I’m a ROTC cadet going to Ft Knox for OPFOR this summer, and wondered if there’d be a problem if I wore my coyote brown FLC with UCP’s?

Take both and see what your COC has to say. Chances are you will not be in a US uniform anyway so it wont matter.

I worked there last summer. They’ll give you a black or OF top to wear, and cadets from all over the nation bring difference colors of TA-50. You’ll be fine.

Mixing uniform and gear patterns shouldn't be a problem. The Army has a hard time keeping up with their latest changes.
If you're going to be OPFOR, you'll most likely turn your uniform inside-out or be given a costume.
You can only be expected to bring what you've issued.