i don't see a problem with adding a boy to your girls. i have two of the tiniest little girls in with my three fat lumps of boys, and they not only hold their own, they rule the cage!

do you have a seperate cage to house him in while he adjusts, and during the introduction process?
is this boy your looking at a single ferret? has he always been a single ferret? do you know if he is social with other ferrets? what will you do if intros don't work out? what if he doesn't accept your girls, or visa versa? are you willing and able to provide for two seperate ferret groups? if he has to remain a single ferret, will you still be able to give him all the attention, love, and play time he needs seperately from your girls?
that would be my only concern with adding another....