Thanks for your replies, I find them all very interesting.
Yep, Tonberry I agree that poly/poly is easier than mono/poly and I'm going to give that route a try.
As for the downsides:- My poly partner (who I've only recently educated to the point that he realizes he is poly)felt a great sense of loss when he got married (not to me) and had to give up all his female friends. That to me is a downside, in that our society, hasn't recognized polyamorous people as legitimately having different needs from monogamous people. Many of you have had years trying to fit yourselves into the monogamous norm and experienced lots of pain in the process.
Due to a shift and the fact that he's working from home he is now trying to make some new, local, female "friendships" while being honest about his poly nature. It isn't easy and I see his frustrations. That is another downside.