in a 20 galllon i would add no more than one or 2 smaller fish, maybe 3 if they are real small species, and figure the adult size of the fish, not the purchase size, you do have too many, IMO,
the clown and maybe the gobie, depending on the species you have, is where i would stop
and PLEASE DO NOT ADD 2 TO 3 INCHES OF FISH PER GALLON!
that is waaaaaaaaay overstocking, maybe ok for fw, but in salt, the general rule is 1 inch per 5 gallons of water(figured on the adult size, mind yuou)
the best guide is the ability to maintain stabel water qulity over time, but it is generally close to one inch per five gallons
HTH