Your hive fleet must have at least 4 species: one from each deployment slot type besides Fast Attack. If you do choose to do this, one of 4 can be a mutant. You are allowed to give mutant Gaunts Venom Cannons, etc., but each mutant can only choose one of the choices. The points cost for weapons-beast should be in you Codex.
A word of advice: Usually Hive Node is the only really useful mutation.