Depending how hot it is, you may not need a flash pass. If I remember correctly from the TPR tour, most of the people were at the waterpark as it was so hot outside. All of the rides were walk-ons. I would definitely wait and see when you get there.
Fiesta Texas is a beautiful park. I definitely want to go back someday. Especially since iRat was closed during our visit.
Out of all the coasters besides iRat, I would say Goliath (one of the best batman clones), Superman, Poltergeist and Road Runner Express (a fairly intense mine train) are not to be missed.
Also do not miss the "Texas-themed" laser show at the end of the night. It's really good.
Have fun!