It's quite common for livebearers to fight with each other, the excepted ratio to even out aggression is 3-1 in favour of females, it's likely your dominant male is just fighting for the both females.
Try upping the female ratio and see if that calms him down, if not it would probably be best to keep them separated as they will often fight till the death.
As to the plecs, Betta fish is probably right and BN's are renowned for ease of breeding and she is also correct, BN's and commons wont cross breed, infact havibg commons breed in an aquarium environment is very very unlikely.