Having a kabbalas shabbos minyan is like having a circle with four angles equal angles and sides -- a halachic contradiction in terms. The gemara has two views as to the proper time for mincha/ma’ariv: according to one view mincha must be completed before plag and ma’ariv may be done immediately afterward; according to other view mincha may be davened until shkiya and ma’ariv only afterward. Either view may be followed provided one is consistent. The problem with the 7:00 minyan is that it takes the short end of both sticks -- mincha is later than plag but ma'ariv is earlier than shkiya.
Someone commented that there was an easy solution: daven mincha early, go home, get ready for Shabbos, make kiddush, eat your meal, and then come back to shul and daven ma’ariv at the proper time. Everyone is happy -- the kids get supper at a decent hour and you avoid the contradiction in zmanei tefilah.
Only problem is that the GR”A in Ma’aseh Rav shoots this down. GR"A holds that kiddush may not be recited before ma’ariv. The proof: the gemara (Brachos 27b) writes that Rav davened ma’ariv early on erev shabbos. Asks the gemara, did Rav also say kiddush? The gemara answers by quoting Rav Nachman in the name of Shmuel that one can daven early on erev shabbos and say kiddush as well.
What exactly is the proof from this gemara? Just because Rav davened and then said kiddush does not mean that it cannot be done some other way!? The meforshim on the Ma'aseh Rav are so troubled by this issue that they suggest that perhaps the GR"A never said this chiddush in the first place (an easy solution!). You can nitpick and say that since Shmuel first repeats that once can daven early, which was never under debate, and only then tells us the chiddush that kiddush can be recited, he implies that these two facts are related and tefilah must precede kiddush. Still, far from compelling. Even if you kvetch out some diyuk in the gemara, how do you explain the lomdus? Why is it that tefilah must precede kiddush? I don’t know the answer. For the record, the Aruch haShulchan has no problem with kiddush before ma'ariv. But for those who are concerned for the GR"A's view, the very fact that the GR"A thought this chiddush was correct gives it a stamp of legitimacy, even if the proof is unclear (as noted by the meforshim on the Ma'aseh Rav).