The altitude of an airbattle aren't laid by any law, agreement or common rules. It is caused by the possibilities and willingness of the participants and as both the Zero and the Wildcat could manage 33000ft I can't see any reason why it shouldn't have been btought to the top and I've noticed that a try on outclimbing the Zero was a death trap as the Wildcat would stall before the persuing Zero, hence read between the lines: The fight above the Pacific was(!) done above medium height!
But in practice... not so much.
Carrier warfare was typically fought below 20,000ft. The Japanese strikes at Midway, for example, came in at 12,000ft and 5,000ft.
The only high-altitude missions in the PTO were recon (love that Dinah) and the B-29 raids on Japan (and we know the trouble the Japanese had intercepting them)