They don't always approve it and it may not be covered by the scholarship when doing it in the summer.Yes; I know many STEM people who have done this with their gen-ed requirements for their NROTC school, doing these courses at their community college during the summer. They have to get it run by the unit but it's always ok!
Another interesting aspect... you can submit a waiver to the NROTC unit during your senior spring to take one or two course reduction if you have extra credits. It's not always approved, but I've seen 2-3 people do it during my time in my unit