I'm glad you got some medication from the vets, hoping they help clean up the infection.
Yes it is recommended when first bringing a hedgie home to keep changes to a minimum. Using the same food is the most important. I could be mistaken but I think Meow mix is a low quality food?. If you're going to feed cat food, it is best to feed a premium quality one (one bag last 3-6months so it's not too expensive). Their is a recommended food list on here if you do want to slowly change her food (I'd wait until she is better before doig so). Or do you know what food the breeder/ store was feeding, since you've only had her a week, it would probably make sense to feed the same food and see if her issues settle down. See if the medicine helps first.
I'd also check with your mum what the detergent is. It could be irritating her skin. If you can't change the detergent then I recommend switching to paper bedding.