First problem is the tank size. The Congo Tetra and Rainbowfish are shoaling/schooling species so they need a group, and we now know that this means in the 10+ numbers of the species. Ten Congo Tetra would in my view be inadvisable in a 40g breeder which is 36 inches/90cm in length. As for rainbowfish, it depends upon the species, but some are just as large as the Congo, and some of the smaller sized species are somewhat "delicate" so to speak, and the large 8-9 inch Acara would be an issue.
Most smaller fish will be shoaling/schooling so the tank size will play into this for some of them.