All those would be okay in a 15 gallon? I already have 12 neon tetras and 2 dwarf gourami's so one small catfish would probably be the limit as far as bioload. Probably should have mentioned that in my first post.
Definately otocinclus (otos) and pygmy corydoras (cories) are the most commonly available, and commonly suggested and successful catfish in small tanks. All are reportedly more active/visible/calm with more than one of the same species in same tank.
The oto is your algae eater and I recommend getting two or three for a 15g tank. The salt and pepper cory (C. habrosus) and dwarf cory (C. hastatus) are only slightly bigger than the pygmy cory but are usually harder to find at the LFS IME. I haven't kept any of the latter but I have kept the former and they weren't as active as the pygmy cory.