In my opinion a 29 with adult firemouthes in it makes adding more fish a bit of a stretch. The parents are awfully small for firemouthes too. At least, I thought they got about 6 inches? Of course, I find my biggest problem with getting fish is a reliable source where I can get a large variety of healthy fish, as opposed to fish that die well within the guarrantee. So you already have some good healthy fish.
If I were you I would trade in/sell the young firemouthes, and the adults as well if you aren't planning to get semi agressive tankmates. A free bristlenose is an awesome deal, they're tough, easy on the eyes, love algae and breed readily in material as cheap as 1.5" PVC. I would get a couple more, or if it's old enough to be sexed, a mate. The young are easy and fun to raise, and easy to sell.
As for colourful fish, I love my ember tetras, and they've been through hell. Not even 1" long but taller bodied than neons and the like. They kind of glow like embers, which is obviously how they got their name. Quite cheap as well. I seem to never see embers any more, only flame tetras, which are vaguely similar I believe. Sharper colours, more silver to them, a bit larger. Microrasboras are nice, I think they're supposed to fair pretty well but I haven't got any experience with healthy ones to back that up.
As for cories, peppered, bronze, sterbai, panda, any of the species sold under "leopard" are all hardy fish (if you can find healthy pandas that is). Pygmies are nice too, but are very shy. Other bottom feeders you might consider are upsidedown catfish (who get about 4") or raphael catfish (you wouldn't see them much, but they're some of the few bottom feeders that don't need schools. They may snag small fish though). Cherry shrimp are nice, small and colourful. A bit too small for firemouths, but fine with anything else. As there are probably more people breeding them than there are not keeping them, you'll find them for sale in the classifieds ranging from 0.50$ to 1$.
Good luck with your tank, make sure you plant it and don't put in a Harry Potter figure that can leak toxins in and kill your fish
