I just spoke to an ex PA Res Supervisor from the 1960's-70's who worked at various overseas stations and their 100% for real expert opinion is there were no circumstances you could fly purely domestic on PA before deregulation, with the exception of Honolulu. or to US Protectorates like Puerto Rica and Guam, unless you were staff in which case you could get away with it.
After deregulation but before National PA did sell seats aboard domestic legs.
Not my opinion, but the answer of someone whose job it was.
On the fleet size question, towards the end of the 1980's-early 90's the fleet was approx. 30x 747, 90x 722, 20x 310, 13x 300, 5-10x 732. Obviously fleet numbers were not static, but that's the numbers roughly over the period. Not so hot on Pan Am Express but I believe those numbers have been quoted already.
People wondering what would have become of PA, almost the entire 'old' pre National PA still exists, just look at UA Schedules across the Pacific, to LatAm and ex LHR, Delta ex. JFK + Shuttle and Lufthansa ex Berlin. The only bits that don't exist are the CArribbean, PA Express and Miami Domestic flights (of which not many were left).
Hope that helps