I have a JBJ nanocube, it's inhabitants are 1 false perc, 1 featherduster, 1 firefish (goby), assoreted snails, 1 scarlet hermit crab, and all are doing well in the tank.
Now Im the first to admit that the firefish is afraid of the clownfish and will often jump about a bit, but thats to the hood on the JBJ nanocube it prevents him from becoming a freeway jumper. But he spends most of his time just hanging out in his own little cave.
I dont think I'd put more than 1 or 2 in a 20 gallon tank, really thinking about it, I dont think I'd put more than 1 in a 20 gallon tank. But it depends on filtration, bioload etc, if that is met then I think that 2 would do okay in a 20 gallon assuming that there is enough hiding places and "places" to be called their own.
But hey, thats Just my opinion.