I agree with the others - you shouldn't have a problem, at least in the short term.
I've been told that the system first looks to see if you have an on-site reservation. Length doesn't matter - it could be for only one night. Then the system allows you to make FP+ selections for as many days as you have admission. (However, there is a limit for APs/PAPs, I think it is 7 days, but not entirely sure about that).
This may change to stop people from making on-site reservations they don't intend to use. There are reports of guests making a one night reservation (often for a campsite) to make FP+ selections at 60 days. Either they cancel the on-site reservation after that or don't use it. (This is very upsetting to the regular campers who cannot get campsites due to that practice. I suspect they are vociferously complaining).