no gabrielle, she's too young and the "oh she's a veela so older than she looked" thing is overdone and kinda silly most of the time.
i don't get the only 1 patil twin, for 1 good reason, no matter how good of an actress, if 1 twin suddenly becomes totally devoted to harry, the other would notice the change, and harry needs to keep this as quiet as possible.
susan bones is a good addition strategically, for her ministry connection, maybe help him get a bit of legal protection from all those fucking with his life.
So i'm cool with your current choices, but think the final 2 ought to be either both patils (think of it, twins are awesome, add in tonks and it could be TRIPLETS!), or perhaps hermione and a professor to get some staff protection, shield them from snape and so on. A professor seems a definite choice, but if not hermione for 7th, then i think a lesser used character but one that can plausibly be smart/useful, unlike gabrielle, or lavander, one is too young, the other is an airhead, better slave material than bonded. His choices for the SSS should be based upon knowledge, intelligence, skills, and power, both magical and otherwise (political, financial...), not just on how hot they are, because there are tons of hot women around, and he can always enslave the hottest that don't meet the other required needs. Can't enjoy the benefits of the bonds forever if voldie kills him because he bonded hot, but dumb and/or weak witches.