Hey friend,
So.. 7 plants is a lot, but really its all what you can handle. I know some people who run a perpetual auto harvest, where every 3 weeks they harvest 3, and germinate 3. I'd say to get that 60x60x70 up and going asap, it's the best way to keep a consistent and comfortable environment. There shouldn't be a problem fitting all 7 autos in there and then you'll know how much you want to run on future grows. I can run 3 in a 4x2x5 so you shouldn't have a problem with 7 in a 5x5x6.
Good luck with the grow.✌️