SOme sevs, once adult, will simply not tolerate other severums in their space. Even if your two larger sevs are a M&F, there is no guarantee whatsoever that they will pair off. Sevs have a mind of their own as to which of the opposite sex they find attractive, or not.
My advice is to decide whether you are happy with only one sev or not. If you are, then keep the large bully and rehome the others. If you want multiple sevs then you need to rehome the bully, and maybe even others too and continue trying to find sevs that will tolerate each other. It's unpredictable. As you probably know, sevs become territorial very quickly about the tank they are living in and become less likely to accept others.
I can strongly recommend something like a pair of uaru to go with your sev. Large angels also go very well with sevs.
Good luck