This is (IMO & IME) a very "iffy" area. Some paired clowns will not tolerate any other clowns, period. They are more territorial than aggressive, but can easily claim an entire 90 gal tank as their territory. Smaller species of clowns like Percs or Ocellaris may do well, but may not. Mated Maroons, Tomatoes, and other large Clowns will almost never allow new clowns in the same tank.