1. As long as you keep stress levels low, then you should be able to feed her the first Sunday. In the first 2-3 days you'll usually have trouble getting them to eat, but usually by 5-7 days they'll be more comfortable in the new setup.

2. I wouldn't recommend feeding pups when she's still only 75g. Pups can get up to 30g so that'd be almost half her weight, plus it's typically better to feed smaller rather than larger when you first get a new snake. Does the store carry mouse hoppers or weaned mice? Those are about the same size as a rat fuzzy so could be used as a replacement. Another alternative would be to feed 2 rat pinkies if you'd rather stick with rats. Lastly, if you're picking her up in person you might be able to get the seller to give/sell you one or more of the fuzzies she has been eating to keep her diet as consistent as possible that first week.