I'm thinking of using this sump. please critisize me and tell me how to improve it. I want to use it for a bimac

A ten gallon tank would work as a sump for a 15-25 gallon tank...I'd bump it up to a 20 gallon sump, and a 50 gallon tank.
Don't forget the skimmer, the sump is the best place for it !

fish in the sump should work. of course don't overdo it. but one or 2 damsels, clownfish,... anything small should work fine.

I used my sump for some small fish that were in the tank when starting it up. it was also a great place for dumping surplus feeder crabs

just make sure skimmer, pumps, ... are inaccessible to the fish/crabs/...