A dragon goby when full grown should be in a 75 at least. A 55 wouldn't really provide enough turning space. And if he would spaz out for a second, he would hit himself on the glass causing more spazzing until he realized where he was. Mine doesn't like any surprizes at all lol.
Dragon Gobys do okay with aggressive fish that do not actively pick on fish. Mine does fine with a purple spotted goby.
For a 20 gallon, I would suggest something that maxes out at up to 6" and then a couple smaller fish. Mine looked great with my baby whale mormyrid, betta and guppies. As for a pleco, get some kind of rubberlip pleco or a bristlenose pleco. They both stay small enough for a 20g.