Your generic clownfish (A. ocellaris, or A. percula) aka, the ones that look like Nemo can be and are very hardy.
Clownfish are not usually open water swimmers, they like to host, especially with an anemone, so a smaller aquarium will suite them just fine.
I had my pair in a 14G for quite a while before I moved them. They could have stayed in there, but I wanted to get them in an anemone and my 14G didn't have the proper lighting for one.