Three-spots can get up to six inches, but that's not common in tank-kept species. Still they can get big. A 29 might be a little crowded for two, but I tend to be conservative in that regard.
Definitely consider 5-8 corys, depending on size. You'll definitely get more interesting behavior. To be honest, I'd look more a the smaller gouramis, such as the honey dwarfs, and then maybe a nice top-level fish.
2x20-longs, one heavily planted. Platy rancher. They. Won't. Stop. Breeding.