Speaking as a client, I'd say that certain pros would be veryvery good at making this a great experience, while other pros could be disastrous. So I'd encourage screening potential pros by experience and reputation first, and rather than using "type" as first criteria. And though it's not the same thing, when I've fielded invites (non-professional, mind you) to join as a third, I've always tried to be clear that both partners were into the scene. (Not, as happened in one unfortunate encounter, with one was "just going along" because the other clearly wanted to do this.) And, yes, your partner gets to set the boundaries and you get to have fun within them.