If I use an 8 speed bar end shifter with the TT bar on my Allez, will I need to change any other parts of the drive chain?
Pretty much any 8 speed Shimano-compatible bar end shifter will work if you're running a Shimano drive train (If you're running SRAM, you'd obviously need a SRAM compatible one). 

Those will work, as will the ones from Microshift 8 speed among others. You'll need to re-run your cables, obviously, though.