Not sure if I understand your question.
At Sci-Fi, no matter what you order you cannot use a snack credit to pay for it. Even if it is a shake.
It will either be a TS credit or paying OOP.
Now if you were asking if you could go in and just have something small like a shake and then leave going to eat lunch at a CS then yes you can do that. But again, Sci-Fi would be OOP.
Have fun.
Oh and it is ok food, nothing great. We did it last time because I had to experience it. Was glad I did but won't be returning.
Chicken sandwich is pretty good, onion rings as well. But nothing worth writing home about.