Could be asking for trouble adding a smaller one, but it might not happen (a fight). I added 2 then added the other two (mated pair) in about a week. Now, the female has taken to ALL 3 males !!! She pends the night in a different hole every night,.....should I be charging a room fee ?!?!?!
Anyway, there is really only 1 way to find out. Just like anything else in nature (or tank raised nature), some will be calm, and others will be an a$$. Be prepared to find another home though, just in case.