Of those 9 species, only 3 are available in the pet trade: the peachfaced, masked, and fischers. My AJ is proof that differing species can breed--his father was a peachface and his mother was a masked.
Just because different species can breed, doesn't mean anyone should be doing it. Because we cannot import any more pure, wild lovebirds into captivity anymore, our gene pools are limited. By mixing species, you are lessening that already small pool. Don't get me wrong--there is NOTHING wrong with hybrid lovebirds--I love my little AJ to death! Just people should not go out making more hybrids...especially with the shortage of homes for all birds.
Lovebirds can be very aggressive and territorial. In my opinion, they should never be housed together unless they are a bonded pair. Even then, you hear about the occasional "accident." This is especially true when mixing species...it just is not worth it. In addition, lovebirds are very high energy and need large cages. To comfortable house more than one, I would only feel that they had enough space in a large flight cage or an aviary.