Info needed: why mice? You're gonna wanna switch over to rats for sure, more fatty and filling at that size! Also, are you sure the girl was eating mice when you got her? If she was on rats/something else she might just be picky, but I wouldn't worry about it too much, she might also be settling.

What's your setup like? Also, snake pics? How do your snakes' sides look, how much do they weigh? Mice might be too small, too. Sorry for all the questions, just wanna cover all the bases!

Don't worry about aggression. Some snakes take a while to tame down and that's probably not on you, give it some time and he'll probably chill out. Also, if you didn't buy from a breeder and picked up your snakes rehomed/from a pet store they might have not told the truth about how nippy or feisty they are. I've seen plenty of people try to unload temperamental snakes with little fibs, lol. Don't get scared off if it's just his attitude... I've never met a ball python that doesn't end up super sweet given time.