I would not have thought the shark would bother the birchir, I don't know about the other way round, as I understand it, birchirs will eat other fish - never kept them - ask on oddballs.
With the shark, some are very placid, others very aggressive towards anything they think looks like another shark. If you happened to get a myopic shark that thought the birchir looked like a shark it might react of course. There is a good deal of indiviuality when it comes to sharks.