1) Harem, no removing the male. Usually 1.2 to 1.4 groups.
2) Anything acceptable size for rats is usually good, the extra space compared to mice will help territorial aggression.
3) 6f is good.
4) No experience yet here, but for the most part, you will have problems going from ASF's to mice/rats, and not the other way around.
5) About the size of a medium rat full grown, but they grow slow.
6) Ours are aggressive little buggers, and one got out the other night...big hassle.
7) I would suggest against a typical bin/tub setup unless you can provide a wheel, lots of chews, toys etc...if they are bored, they will chew out. We use lab tanks, and it works out really well.
Good luck