Will a buffer tank used with radiant heating that has glycol in the system ever rust? My current tank is 8 yrs old. I plan to check the anodes

Yes, but quality of water and tank material will determine how bad this situation will be. Any ferrous material in hydronic system will corrode/rust relatively quickly if PEX without O2 barrier was used.

The tank isn't rusting yet and Pex with O2 barrier was used. Glycol is used in the system not water. Would glycol alone eventually rust the tank?

You can't run 100% glycol, so some water is always used. And you can only use propylene glycol NOT ethylene glycol. Glycol significantly reduces heat transfer and significantly increases pumping requirements so it is best not to use any glycol unless you have a PEX freezing risk that can't be mitigated by other means. BTW, if the system is designed properly, you shouldn't need a buffer tank either.

Would glycol with some % mix of water eventually rust the buffer tank. This is the main question I am trying to get addressed.

Yes, if the buffer tank is steel, it will eventually corrode/rust and fail...with only water or with water/glycol mix. How long that will take depends on the quality of the water and on the wall thickness of steel buffer tank. No, if the buffer tank is stainless steel.