I have 3 panda cories 2 bronze and 1 bronze longfin in a tank with tetras and barbs. I have been trying to get either species to breed by following the advice i read (weekly 50% water changes with lowered temp). Ive been doing this for a few weeks and they still arent showing any signs of breeding ...

Dont put cories in the small tank, get a 10 gallon.
For the small tank you can put either a betta, or if you dont want that, get a dwarf puffer, its a solitary fish, its intelligent and its fun to watch, best of all its max size is one inch, but remember that its a carnivore, like all puffers.