Excellent advice. I know of people who are happily poly, but I also know of people who seem to have drama drama drama all the time. (Not that there's not drama in monogamous relationships as well, the poly just seems to add a whole 'nuther dimension to it.) One thing you might ask yourself is, have you had any urge to poly yourself before he brought this up? If not, then before you agree to it I'd suggest that you really think about why you're agreeing.

Also, you mention both having a third for play scenes and having a live in slave. These are very very different things. If you and he think that playing with another woman would be hot, by all means give it a shot. But you don't need a poly relationship to do that, you don't need another woman in your household. If you and he want a "second girl" in your lives and in your hearts, that's a really different matter.