I have had the birmans just over a year now and only in the last couple of weeks has Misa stopped hissing at them.
Every thing isnt perfect but it seems that eventually all four have found a way to live in the same house. Sasa who is a tortie has been here with Misa over 18months now and I still cant touch her, I also wanted a lap cat and neither Misa or Sasa are. The only lapcat I have is Franta.
What problems do yours have, are they actuallly fighting and hurting each other?
I think a behaviourist is a great idea although some of their ideas are not always to ones like and sometimes badly affect the other cats and you.
If your cats are not fighting and its just looks, hisses or avoidance of each other, I would just be patient cos eventually it seems they find their own way of dealing with things, even if it is just avoiding the one they dont like.