this could potentially happen people have succesfuly kept gobies and jawfish but its about 70/50 % that they would fight
i personally would either do
a pair of common clowns (they should be kept in pairs as its not healthy to keep a fish that pairs off and lives in anenomes on their own)
1 yellow headed jawfish
1 firefish
1 Swissguard basslet
5 fish wouldnt be really overstocked as a normal 20g is 2.5ft so really theirs plenty of space
or
a pair of clown fish
1 watch man
1 catalina gobie
3 chromis
but thats just me i do like to overstock a bit nothing rong with that really as long as you dont slack with maintance too much