Where did you find those physical requirements? Very different than the ones in that link I guess they've changed in the last few years. As far as staying 20 years, say I did TACP for 4-6 years (at which point I'd be 24-26), decided I wanted to stay in the Air Force, but at that point I want to have a kid, spend more time home, ect, and as a result wanted a safer job would it be possible to cross-train or reenlist in a different job? If so that sounds like a pretty decent plan to me.
Those are the current requirements.
TACP PAST (Standard and SOF) | ShadowSpear Special Operations
Also, if you follow the links from romad.com they can be found on the specialtactics.com forum.
If you go TACP for 4-6 years you will have only been operational as a JTAC for less than half that time, chances are they won't let you cross train into something else because they have so much time and money invested to get you to that point and TACP is critically manned. Also, I personally can't imagine going from TACP into some regular AF gig. IMO, that would be a major step backwards. Remember that the majority of the AF is there to support the mission, the mission being flying planes, the prevailing attitude being that if you ain't a pilot, you ain't sh**. There is one AF job that doesn't fall into that category and that is TACP - they exist to put bombs on target, using air assets to pave the way for ground forces to advance. Mighty hard to go from that to being another face in the crowd in supply, truck driving, security, cook, mechanic, whatever.
However, if that is what you wanted to do it may be possible, if not after your first tour probably after a second. Also, if you excel at what you do you could maybe get a position as an instructor at the TACP schoolhouse. Talk to your recruiter, I'm sure he can give you more info on cross training.