I primarily use green gas (ASG Ultrair) due to lower temperatures in UK. Abbey ultra predator if really hot day or 144a predator gas.
Yes stick with TM mags. Both 4.3 and 5.1 will fit, 5.1 having 3 more bbs and longer base plate
Saw Fobus 1911 that apparently fits hi-capa but I run the nuprol one albeit its designed more for the 5.1 length. At least with nuprol you can adjust the angle which i think you can't with fobus. Same price as fobus around £20