I do not believe you can retire off AD and then draw a paycheck from the Reserves (will do a little research and get back to you). Is the full-time portion as an AGR?
The 28 yrs rule also applies to the Reserves, but, as usual, a waiver can be granted if leadership feels they want you to hang around (we currently have 2 in my squadron, 1 full-time and 1 part-timer).
I am sure there are other pitfalls, but cannot think of any off the top of my head.
Edit: added AGR question