Although clownfish are in the damselfish family, they are quite peaceful compared to some damsels. Also some clownfish are more peaceful than others. The common clownfish (like Nemo), Amphiprion ocellaris, is quite peaceful and only grows to about 3inches. This would be fine in a small tank. Other clownfish like the Spine cheek clown, Premna biaculeatus, or the tomatoe clown, Amphiprion melanopus or A. frenatus, are highly territorial and will often kick hell out of other fish in the tank. They also grow to about 5inches and need a 3ft tank or bigger.
Even in the damselfish family there are peaceful damsels like the chromis and small yellowtail damsel, and agro ones like humbugs, Sargent majors, and blue damsels.
Tank mates for clownfish can include blennies, gobies, small angelfish, tangs, small wrasses and anything that isn't gong to bully them. In a small tank (10g) look at small gobies and blennies, pygmy/ fairy wrasses, and a few shrimp or small hermit crabs.