Yes, inter-service training happens.
No, it isn't as simple as just wanting the training, you have to qualify, have a valid need, and be lucky.
Yeah, pretty much; plus they get a badge that their buddies don't have.
For example, if a Navy SEAL wants to train with Marine Force Recon or the Army Rangers, can they?
If they can, what happens? Are they just a better trained SEAL, what.
Only answer if you know, please. Thanks.