Miss 7 will be fine once she works out that she's going into grade 4 with another little friend.
Skipping into a grade with a friend can work out well or it can backfire. In my case, I skipped into a grade where a good friend already was. But it turned out that she really liked being the older, bigger kid and my arrival in her grade threw her for a loop. It also disrupted her social circle since she did different things at school with her friends than she and I usually did together. In fact, I think it was, in part, her resentment of my being skipped that made my skip so unpleasant since she essentially blackballed me with her friends.
If we had had a deeper friendship, I can see that it would be a really good thing. But I did think it was a deep frienship until the skip when it all fell apart for me. Since it was a small school, there weren't many other options for friendships, so having her ditch me was a big deal.
I don't mean to suggest that this will happen or that your DD will experience the same social disruption I did. But I do know that there was no one looking out for the social aspects of my skip and I was left to try to handle things on my own. I suspect if someone was aware of the problems for both me and my friend, and addressed them with us we might have been able to salvage our friendship. So I am hoping that my experience will help you keep an eye out for trouble spots and head them off early.