There is no reason you can't use your 6wt single-hand rod for summer fish. Most of us who have fished for steelhead for more than 20 years have used 6 wt. single-hand rods for summer runs before the 2-handed rod revolution for steelhead. Just don't use a trout size tippet.
One thing to keep in mind also is that with today's rods, a rod labeled as a 6wt probably isn't really a 6wt. Fight fish with the butt, don't be afraid to really lay into them.
A rod being "noodly" doesn't reflect the amount of backbone it has. I fish primarily glass for just about everything (6-8wt for steelhead). Most people at first glance would think that they wouldn't have the backbone for large fish, but that's where the real power is (butt). You just can't be afraid to use it.
As stated, just don't used trout tippet. I'm always blown away by hearing of people taking 30 min. to get these fish in. It doesn't take much to play them fast and hard, get them in in under 5 min, slide your barbless hook out, and they'll shoot off full of energy.
Yes, use heavier tippet and also the butt section of your rod for power, as is suggested by other forum members here, and you should be fine with your 6 wt.
I used to see some of the local, cash-strapped youth fishing for salmon in the rivers here with trout rods spooled up with 20 lb test, or so. They couldn't yet afford to go out and buy proper salmon sized gear. Those kids landed 'em.
Due to "tennis elbow" having developed in my casting arm, I have quit using my 8 wt single hander, probably forever. My 6 wt is as heavy as I will go with a single hander, when I get back to casting something that heavy. I'm still avoiding it. The tendon still cries out after working out briefly with my 3 or 4 wts, but that's what ice is for.
I might opt for one of those 5 wt switch rods one of these years, if I want to seriously get back to swinging flies for steel.
I used a 4 wt for trout/src, and a 6wt for steelhead when I first moved up to Washington from California. Why you may ask? Well the simple answer is we just used smaller flies down there. The river conditions dont require an intruder most of the time. A small fly does a better job when the rivers aren't looking like your morning cappuchino. People used to tell me I was killing fish. I would just tell them to learn to fight em right.
The rod weight usually isn't the weak spot. It is the leader or the drag, or the fisherman 99% of the time.
Something to keep in mind for fish that are to be released it is best to "break their will to resist" rather than play them to exhaustion. See nearly as many folks babying a large fish to exhaustion with 9 and 10 weights as 6 weights.
I have caught my share of steelhead on both gear and fly gear and I can honestly say with a 5 or 6 weight fly rod (armed with proper leaders and experience) I can land steelhead as quickly as with gear. I can not recall the last time any steelhead I have caught required to be "revived". If you find that you need to "revive" a steelhead you are over playing them regardless of the gear used.
Once you become accustom to taking the fight to the fish the biggest deciding factor will be what rod will be the best "tool" for the presentation and flies you expect to use.
I'd use the 8wt until you get used to fighting steelhead. Like others have said, use heavy tippet (+/- 10lb) and learn what that much pressure feels like on a fly rod. Downsize when you've got a few under your belt.
There is no worse feeling that not being able to control a fish, when you know you should be wrapping up the encounter. I hand lined in an 18# buck this winter when I was under-gunned. Needless to say, that's the last time I take my 6wt spey rod out for winter fish.
(I believe) if you haven't bent hooks out, on occasion, you aren't trying hard enough.
I also have a rule that if I'm having a tough time turning a fish within a reasonable amount of time (just a couple of minutes), it's break-off time. I've lost a handful of nice fish because of this rule, but I'd rather have those genes in the gene pool than floating belly up 20-30 minutes later.
Single-handed rods: If you're fishing hard current water (big rivers) I don't think a 6 is enough. And with a big fish in small water, you may still not like the 6 (say you really have to put the screws to it to keep it out of a log or whatever.) I used a 6 for a season and a half and then got schooled once, and from that point on started using a 7s. I don't fish big enough water or fish to justify an 8 wt. Point being, the 6 will work but don't be surprised if you find yourself wishing for more rod on occasion, maybe even 1 out of 3 fish.
I kinda like the suggestion of starting with an 8 until you land a few a get some idea of what you're going to encounter. It's important to not overplay any fish you intend to release, steelhead or otherwise. While we all occasionally get surprised on the water (the unexpected monster fish, steelhead while fishing for trout, etc.), I think it's wise to aim for the average and/or have a little margin of error. IMO a 6 is just under, a 7 is perfect, and an 8 would be having some margin of error. But again, if you're fishing fast current water or hot fish (higher water temps) or a run with bigger fish, I think an 8 is a wiser choice than a 6.
I have used a 6 weight single on the Ronde fishing dries and had no problems landing steelhead with it. Now Ronde steelhead isn't like summer steel from a Salish or pen river but a 6 single worked just fine. Again I think a lot of it has to do with who is handling the rod.
Lots of good points being made. It just occurred to me that nobody has addressed what I think might be the best reason to fish a weight or two above the bare minimum: While you may be expecting fish between 6 and 10 pounds, and that may be a very reasonable expectation, Nature does grow 'em bigger on occasion, and sometimes, the occasional freak is the fish that comes to play. When that happens, in all but the very softest of water, you won't be able to turn that fish with a 6-wt. Then, you have three choices, none of which is ideal:
1. Break the fish off. It seems some of the posters here have caught so many steelhead that they don't mind doing this. I've caught my share, and I would still much rather be able to land that fish. That way, I get a close up look at a cool fish, and I get to keep the fly that fooled it.
2. Break your rod, which may be fixable but will likely never cast the same again. When you start hearing a creaking sound, this is about to happen, and it's not cool when it does.
3. Pull as hard as you can without breaking your rod and wait for the fish to exhaust itself. If it's a wild fish, it's as good as dead. If hatchery, its flesh will be considerably less tasty, due to acid buildup.
Why leave yourself open to making this choice if it's not necessary?
For the record, I'm not suggesting that those saying a 6-wt. is sufficient are wrong. Many of them may be more experienced than I am, or else they may have the healthy attitude that a hookup is cool enough without dealing the deal every time. Only speaking from limited, personal, subjective experience here.
I don't know anyone that invests the time and money into steelhead with the attitude of "whatever.. I don't need to land them".
I know from doing it lots of time trying to turn a fish before it hit the chute below the riffle that I can snap off a steelhead with 8lb maxima at will with a 5 or 6 weight. You can't pull any harder with an 8 wt, instead of a 5 or 6 wt, if you can already break the leader that most guys are using for summer fish. (I bet if you use 10 lb maxima you can turn the fish or rip the hook out or break almost any 8 wt)
You guys are totally nuking this thing out with internet logic... Just go fish, put good side pressure on them all the time, and get them in with the rod you choose to use.
Yeah, what Jeremy said. Technically, altho the rod is handy, it really isn't necessary for playing a steelhead. Knowing that 8# Maxima can land a 26 pound steelhead in 25 minutes, fish with confidence with the rod of your choice. If you don't land the steelhead, it isn't because of the rod you're using. That said, I do prefer using a rod for playing steelhead, and I like 7 & 8 wts in single hand rods.
As long as you don't try to lift a large fish with a 6 wt, you will not have any problem using one for summer runs. Heck, you can land 35 lb kings on a 6 wt if you don't use the rod to lift the fish. Simply point the rod at the fish if it is a large one, have minimum bend in the rod (i.e. you are not lifting the fish with the rod), and play it off the reel and you can land fish in a decent time without exhausting it.