First, welcome to the hammock forums. In response to your questions.
1. I don't have any experence on a MSB on a gathered end hammock. But, my suggestion is start our with an 11 footer. Once you spend a little bit of time in it, you can always trim it down if you want it shorter. If you go the MSB route, I'm guessing that any weight savings by trimming down the length would be off-set by the weight of a MSB.
2. For my kids, my 12 (about 5' 1" tall) year old has a Hennessy Scout which is about 8.5' x 4'. He also has a 9' gathered end DIY. Personally, he likes the full 60" width of his DIY because he folds the excess over himself.
3. Im here in Georgia. For three-season camping, a blue walmart pad wil be all you need for bottom insulation. Duing the early spring and late fall, a 20 degree sleeping bag will serve you well. During the summer/late spring/early fall, you won't need a lot of top insulation. I tend to use a 40 degree top quilt/bag.
I took my scout troop camping several times this past winter (Raven Cliff Falls, Brass Town Bald, etc.). I had a 20 degree Yeti underquilt and a 20 degree overstuffed top quilt. I layered up my clothes a bit and I was warm and toasty.