DD only started at SHSG at the very end of year 7 and I know that she is in a form with girls not from the area we live in, but I think
that on the whole they do group the girls geographically. I would suggest that you contact the school if you have particular reasons for your DD not being allocated to a form in a particular way, and see what they say.
In terms of setting, at least in year 8 this only seems to be for Maths and French - I couldn't make any sense of some of DD's class labels when she moved up last September, so I asked. (I had asked DD first, but this didn't get me very far - it turned out that she was in a middle set for both, but she was subsequently promoted to a top set in Maths
). There is some testing at the beginning of year 7, which I believe is for setting purposes.
I wouldn't worry too much about your DD's form allocation - in a form of 30 and with another 120 girls in addition to this she will soon make new friends.