As an Army Reservist, if you fail AIT due to APFT, can you join a different service within 6 months (like the Air Force)?

You have to check with a recruiter. They would know best. More than likely you got a failure to adapt discharge which is neither good or bad. Depending on the branch you choose to go into, a repeat of basic is not likely.

You are most likely looking at an RE3 with the reason being APFT failure. That would require a waiver. I can almost guarantee you the AF will not entertain the idea- they almost never give waivers to enlist. The Navy might consider it, but not likely. The army would require you to take and pass a full APFT before even entertaining the idea, so I imagine the other services have a similar requirement.

If you did get in, yes you would need to go through that branches IET.

Recruiter from a different branch may not even want to touch this one.