I think you would be fine keeping a fuzzy dwarf in a 29 or 30 gallon aquarium.
A clown and blenny would be fine as long as both are a bit larger. I used to have my fuzzy dwarfs with a lawn mower blenny and they never even noticed him. The fuzzy might go after smaller clowns, but something larger like a sebae, tomato, maroon, or larger pair of percs would be fine.
Fuzzy dwarfs are not aggressive, but they will try to eat things that are much smaller than them.