Depending on the size, you'd be hard pressed to find a fish in the sea that won't eat shrimp ;D
I've kept Apogon maculatus and psudomaculatus, both western Atlantic species, along with a half a dozen other varieties of cardinals. Always been a favorite.
A 20g is pretty tight quarters. Flame cards get up to like 4", I wouldn't recommend keeping one in a tank that small.
Maybe a yellow card? (Ostorhinchus luteus). ORA was breeding them for a while, not sure if they still offer them. They stay small, attractive, easy going. Something you don't really see every day. You could easily support a pair in a 20g system.
Red spot cards, (Apogon parvulus) might work too, but I've never found them very attractive, personally.