Your question is pretty open ended.
How good of a shot are you? How disciplined are you in not taking questionable shots? What about distance? Reload or retail ammo? Saving fur or not? Do you typically hunt/ shoot with farm houses nearby or way out in the middle of no where?
I personally would not go smaller than either 20 tactical or 204 ruger for coyotes. Both may be a bit on the large size for fox. especially if planning on saving fur.
Just about anything will work in one scenario or another.