Year 7 from my experience can be a tumultuous year for girls. You get a combination of still very hands on parents, including controlling / helicopter, and parents, who step back completely when the children still need guidance. And new found freedom leads them to becoming a little bit feral.
Secondary is an opportunity for new friendships and the children are suddenly thrown in a situation, where they’re mixing with other kids from very different backgrounds. Coupled with this, hormones are raging and rapid brain development means they make silly decisions and are not capable of thinking things through or looking at things from other people’s perspective.
Your dd will meet a lot of children the pool is a lot larger and what you tend to find is girls tend try on different friendships for size and chop and change until they find the kids they get on with. Then come silly fallings out and tribes, the latter of which are both positive as you feel accepted and negative eg the sporty kids don’t like / mix with the emo kids etc.
My dd is in yr9 and the girls are still incredibly sensitive but they do have a bit more emotional intelligence and empathy is starting to return. Despite dd being well liked and not creating that much drama in comparison to her peers, she still has her moments and I was told she was pretty unkind to another girl at the beginning of year 8 when she thought she was defending one of her good friend’s honour.
My dd’s friend went to a school with one class intake and very few girls. I know the scenario you’re describing. She is the same age as dd and has found friends. It took until now to completely distance herself from the girls blowing hot and cold despite having several other friends.
Your dd will work it out with some guidance from you. Secondary parents really don’t tend to get involved with other parents. If you want to prepare yourself, you could perhaps read up a bit and google books aimed at parents for teens. Untangled, for example, is free to read atm if you have Amazon Prime and covers year 7 onwards.