I haven't read all the responses so sorry if I bring up something someone else did.
I would start with your clean up crew. You really need them in place before you add other fish. Your cleaning crew can consist of catfish, plecos, snails, shrimp, and loaches. Each does something different. Catfish eat uneaten food, plecos clean off algae, snails eat everything that the catfish don't, shrimp handle algae on plants, and loaches eat pond snails. These are broad, general descriptions and there are exceptions. So do your homework and research the fish you are leaning toward. And be prepared to nix your favourite fish because it doesn't fit in your tanks community.