Yes, you certainly can stock minnows in the middle of summer. Don't overcrowd them on the trip from the fish farm and keep them as cool as possible, they'll be fine...
I would try to get rid of them... Green sunfish are usually pretty aggressive. Try a small piece of worm on a small hook on light line... If they are both male and only two of them, they'll eat some of your FHM, but plenty will survive. Depending on the size you get, but if you get a mix, many of your GSH should be too large for a GSF to eat... If they are both males, I almost would be tempted to leave them in there and grow a couple of monsters. They will get giant!
