I'm looking to upgrade my boys rustler t to my old 2.4GHz TX/Rx (old associated) and he currently has a hitec 645 steering servo (analog). Will this cause problems?

The older analog servos can have problems with some of the higher framerate 2.4 radios but I suspect it'll be ok with that one. There will be no harm in plugging it in and if it makes a lot of buzzing noises at/around neutral or has jerky motion, it probably won't work well, if it doesn't, it should be fine.

im using an old $20 "ebay special" airtronics 94358z with a futaba 4pl and no issues whatsoever

Appreciate it folks. Well press on with upgrading his truck!