Both bristle noses and clowns are dwarf plecos, and neither is likely to get to be more than 5 or 6 inches. The biggest issue will be having sufficient hiding places/driftwood to make them happy and allow them to create their own territories.There aren't many plecos that can permanently live in a 20g for the rest of its life since many of them get so large. Plus, they are massive poop machines. If it were up to me, I'd stick with a trio of BNs; I'm in love with BNs.
I noticed that you're in NY. If you're in the city, the LFS on elizabeth st. has some rarer species of plecos. Haven't paid them a visit recently so I'm not too sure of their stock.