I think troutman is right on. I don't think they will marry well. Mallard typically does not marry well to anything anyhow, and is rarely married even in classic atlantic patterns.
I have always cut the mallard from the stem as well, and have been wondering if I'm missing out on something. But I think my wings are okay, and I've seen troutmans, and I think they are quite nice.
Another Idea for adding color on top of a spey would be to add horns of macaw tail or goose. Not necessarily traditional on a spey, but the look nice and are easy to do. And besides, mallard is so fragile that after a few casts, the wings will split and you'll wind up with goose horns anyhow.