The small FDX airplane ( C208 & ATR ) out of Puerto Rico ( SJU for 208, BQN for ATR ) are operated by Mountain Air Cargo on behalf of FDX ( FDX own the a/c ). Since there are N-register and base in Puerto Rico you have to be a US citizens in order to work for Mountain Air Cargo.
Ameriflight also do some run for FDX & UPS out of Puerto Rico using there own airplanes. And Corporate Air fly a couple of Emb 120 for FDX out of BQN.
FDX & UPS use also some other contractor to fly in the different carribean island, Air Tahoma used to do some but went under, and I believed that the run they were doing for FDX has been taking over by IFL.
To make it short every plane base in
PR, you have to be US citizens in order to fly for those companies.
As for Mountain Air cargo, they usually like to take local pilot to fill the position on the C208 and ATR in Puerto Rico, exception happen but rare.
Hope this help,
