Aim for a PH of 8.2 (though a minimum of 8.0 is acceptable)
If you choose common clowns rather than tomatoes, Clarkis or Mariins etc then they are far more sociable. I would advise you buy them small and get a pair. This way you are assured of getting a male/female that will bond.
Although its possible to have more, you will need a larger tank to alleviate territorial problems.
I would say that ideally a 20 gallon tank is fine. However i know some onf here have at least one in smaller tanks.