The shield in ANH was a spot-shield, and it guarded only a certain area of the planet. I would assume that it was like the portable shields the Gungan's used in EPI, except a lot bigger. It was sufficient that aerial bombardment would have been a waste of time.
The planetary shields TRF uses are a network of similar structures, which cover the entire planet. How far the network goes into the atmosphere is debatable. It may have been addressed in previous RP's, and it may not have. I would guess that the shields would cover the smallest area possible, to preserve energy. That would mean not covering all of the outer atmosphere.
The shield that covered the Death Star II was a remote projection shield, not a planetary shield like we use on a regular basis.
Maybe a 'frigate' could enter outer atmosphere, if it maintained forward velocity. But at what cost? Enormous structural strain from the strong gravity, possibility of hull damage from the friction, etc.