How wonderful of you to rescue the poor little sweet girl! Can't talk much as I'm at work, but, yes!, it's perfectly ok if she only eats 1.5 times a day. Some even prefer to eat once a day, as long as their weight is healthy. She may not be eating because she's a little stressed with all the changes. Plus maybe she's not used to it yet. I say give her time to adjust, and lots of lovin', praise, attention, hugs and kisses.
I don't know about the gastro food though, I'm always skeptical when vets give food out. I'm going to suggest getting a high rated food, such as fromm, totw, etc and add probiotics to boost her immune system, and salmon oil to help with her skin. For fleas, someone here suggested to bathe her in blue dawn soap and lather for 10 mins I think, that should help get rid of the ones on her.
Again, bless your heart for taking her in. She'll thank you ten-fold, no doubt about it!
And where are the pics!!!??? You gotta share now that you've introduced her!