Fun thing to do, you can shoot the core out of any ally's power armor without turning them hostile as long as you are in combat.
You'll get mostly T-60 armor doing this, actually that might be all you can get. Atom Cats always seam to be under attack by raiders. BOS show up everywhere after the air ship arrives. Shoot out their cores, if they don't die, you can also get their frames.
The first mission the X6-88, when I arrived at the place to collect the synth the BOS were there. I collected 5 frames, and 1 set from a body. None of them turned hostile.
Also, the frames show as stolen unless you use the console command to make it yours.
Also also, if you have the perk where you can shoot through walls, you don't need to be behind them to shoot the core. (Including with Enemies in PA)