Bead Head Prince, and Bead Head Hare's Ear both in size 16; these are the only two small nymphs I fish, and I do relatively well, if I do say so myself.
Seriously though, it amazes me how few people fish Mega Princes and Possie buggers; if you don't know, now you know. Try either, or better both, in a size 10.
With those four flies, I would argue that you would never need another nymph for trout. They adequately cover the spectrum of colors. You can tie them with Ice Dub, or with natural fibers. You can make them both very dark or very light changing nothing but the dubbing you use. I even tie them all with varying amounts of rust colored dubbing in the fall to imitate October Caddis hatches.
The Teeny Nymph operates under the same principle, and Jim Teeny has made a career of fishing nothing but "one" fly. I prefer the bead head style, and I don't particularly like working with Deer Hair, but really, you could not go wrong there either. Less emphasis on crazy patterns, and more emphasis on color variation is the way to go, IMO.
Ev