Two railroads that come to mind that had F units for power is the WA&G and the WA RR.
As far as I know there is NO rules groven how a cab unit can be used..PRR would assign E units to locals if there was a power shortage..I worked a urban local in Columbus that was powered by a single F7.We reverse moved back to Yard B (located in the Cleveland Ave yard complex) with the shove being protected by the rear brakeman and conductor standing on the platform of the cabin(PRR speak for caboose).