Personally, I don't think cohabitation is a good idea. Do you absolutely need to save money or do you absolutely need to test the milk before you buy the cow? If you two are just saving on living expenses, that's a financial arrangement that can quickly turn into a disaster of a relationship/platonic roommate situation.
As far as his roommate, I wouldn't want to live with two dudes who aren't relatives of mine. Wouldn't have wanted to do it as an undergrad and certainly wouldn't want to do it as a grown woman college graduate with a boyfriend who is in grad school.
Naughty co-eds.
Of course it could all work out for the best. Good luck with whatever you decide. Just make sure that YOU and your boyfriend have no regrets and minimal apprehension with whatever you decide.