Agreed, there is a lot of information out there and it can feel pretty overwhelming at times.
Sounds like she's been on a combination of Royal Canin and Sunseed and you're thinking about Mazuri?
The short version on RC is that ingredients aren't the best and (generally, depending on which version) the %fat is too high. It's fine for the short term, especially for babies who need little pieces and can put those fat calories into growing. The short version on Sunseed is, again, ingredients not the best. It's also not the worst. So, again, it will be okay for the short term. I don't really have much info on Mazuri, so I'll refrain from comment on it.
Given that she's having tummy issues, I'd do my best to stay pretty consistent with her food for the short term. That is, regardless of whether Mazuri is "good" or "bad," I'd hold off on it until things settle down (unless, of course, she's having issues with the ingredients in the RC or Sunseed so that she has to switch away from them). The other thing I would do is get her some probiotics.
Antibiotics kill off the "bad" bacteria that make her sick and the "good" bacteria she needs in her gut to properly process food. You can go to the drug store/grocery store, get some acidophilus, and sprinkle it over her kibbles. It'll help restore the "good" bacteria. You want to give it opposite from when you give the antibiotics... for example, if you give her antibiotics at 9am and 9pm, you'll want to give her the acidophilus at 3am and 3pm. Of course, that's the idea... reality often interferes with that. First, she'll probably be asleep at 3pm and you're probably at work/school/wherever... so I tend to skip that. Then 3am she'll be running and you're fast asleep... so, by sprinkling it over her kibble that she'll eat over the course of the night, you can be fairly certain that 3am is taken care of. There's another product you can find in pet stores called "bene-bac" - it's a gel that contains probiotics. If you really wanted to ensure that she was getting it on time, you could squeeze out a tiny bit (think like water droplet size) and give that to her. Veterinarians can also give you the official prescription version that's like bene-bac in a giant oral syringe. I had that once for one of my little ones.
I hope the antibiotics take care of whatever bad mojo has been going on for her!!