I love Phineas and Margaret - hells yea!
I agree that Gillmore-Russell is a bit strange for the new kids, and I'd prefer for them to just use Russell. I like the idea of Alma being Gillmore-Russell, though, to keep the tie with her kids, step-kids and new kids. :P
Middle names for Phineas and Margaret can probably wait, we don't want to do all the fun stuff now! :)