different species of clownfish should never be mixed. If they can't coexist together in the wild...what makes you think they can co-exist in a small glass tank with 4 walls?
You might be able to get away by mixing peaceful species..however..tomatos, maroons are some of the more aggressive clowns.
I would say the most peaceful clown is the skunk clown. You might like to replace your tomato with a skunk?