IIRC the fuel tank FAA thing is to add nitrogen generators to fill the empty tanks with Nitrogen vs ambient air which could start a fire/explosion. The FAA thing is a direct result of the NTSB investigation results from the TWA 800 incident.
I believe that cargo aircraft are exempt.
The pmUA 752s have PW2000 engines. They are now unique to the fleet. Having such a small sub fleet doesn't make sense from a spares and maintenance perspective even though they are younger than the pmCO 752s which are RR RB211 equipped. I believe that PW no longer provides PIPs for the PW2000 whereas RR still actively supports the RB211.