Would I have room for one extra fish with 11 panda corys and 6 serpae tetras? And if so, can you think of any colorful fish that would make a good centerpiece for the tank? I was leaning towards a dwarf gourami, but after hearing of many of them being ill, I don't think I want that fish.
Possibly. I would personally add more tetras instead. As you said some dwarf gouramis from Asia have an illness which name I forgot which just kills them and is non-curable. I think it was an iridovirus or something along those lines.
Have you thought about a honey gourami? They are the same and I haven't heard of them having the same issue as dwarf gouramis.