2.5/5 Gallon Invert Tank

I am looking into getting an inverts only tank. We currently have a 90 gallon reef, 20 freshwater, 2.5 (x2) freshwater, and 5 freshwater. I am thinking about making either the 2.5 or the 5 into an inverts only tank when the fish pass.

What kind of shrimp would you suggest in the 2.5? The 5? Also, just snails, hermit crabs, and shrimp, or could something else go in there too?