The main problem I see with your stock list is that the banjo cat will eventually get eaten, probably by the poly, or the dat. Also depending on the temperament of the dat it could bully your butterfly fish, and elephant nose to death.
As far as filtration goes, I used to use aqua one canister filters, but since buying a fluval/Eheim filter I have never looked back, and would always personally save for a bit longer so that I could get a better filter.