I have a trio of two girls and a boy and introduced the two girls to the boy with no problem. I also have a pair of boys that I introduced to the trio with no problem... stick with me here :lol:
I think as long as the new bun is neutered (and so are the existing buns) and they are introduced over time in neutral territory you shouldn't have a problem.
I'd say its more down the individual bun character whether they bond or not rather than the sex. Some buns love other bunny company where as others hate it!
Hope this helps!? :lol: