With girls social structure is more fluid than with males... and rats aren't necessarily as rigid as dogs or wolves are either.
I still use the term alpha, although lots of rat owners don't necessarily like it, because it is relevant. But with girl rats it's more like kids at a playground. Yes one is the bigger and the stronger, but that doesn't mean that some times one of the smaller kids might not chase the bigger one for example in a game of tag.
Our Fuzzy Rat was a brilliant rat but she got tumors and was a year older than Amelia who was a culled high white show class rat... Amelia was huge and strong and the old and sick Fuzzy Rat was no match for her... But Fuzzy Rat always stole Amelia's food and treats... And when Amelia tried to fight back Fuzzy Rat would roll over, get her belly groomed submissively and then proceed to run off with Amelia's food anyway. So who was the alpha? The bigger, stronger rat, or the older smarter one? I suppose it would have depended on which rat you asked.
With other girls we have had different dynamics... as Amelia got old an sick, Max got pretty pushy with her... but when pushed too far Amelia would snap back and like I said she was huge. Max on the other hand maintained her position with Cloud pretty much until she died... She was a no nonsense kind of alpha and although she didn't push Cloud around or assert dominance, she wasn't the kind of rat that played either. She was happy to snuggle and preen, but as soon as Cloud got pushy, she's snap at her and say leave me alone in no uncertain terms.
So girls can have some pretty complex relationships... if you look closely you can pretty much tell who has the superior social status and who the alpha really is... it might be the strongest girl, or the smartest girl or even the nastiest girl and it can even change with time and health... but it's not necessarily rigid....
Fluffy, our part wild rat almost tore Fuzzy Rat's throat out, but beyond that one act of extreme hostility she never exerted any dominance behavior, no power grooming, no humping no stealing food... nothing. She just wanted respect, and for sure she got it. Mostly she was a friendly and sweet rat that lived and let live. She was easily the more dangerous rat by a factor of ten... but as to taking charge of anything, it just wasn't something she cared about. When the two rats were out and about, Fluffy would tend to lead, but it was more like Fuzzy Rat would follow her than she was actually leading anyone.
So yes the term alpha does apply to girl rats, but you can't take it as literally as you would with wolves. Like in a group of teen age human girls, if you look closely you can usually make out who the leader is, but you can also bet she doesn't always get her way.
I hope that makes sense.