Fix protocol simply prevents CGminer/bfgminer from attempting to auto-switch any getwork pools to stratum/gbt. I think you would would only use this in special cases where you only want to use getwork. If anything, using fix protocol could decrease your hash rate since getwork is less efficient than stratum or gbt.

And the issue I described with p2pool only applies to BFL single FPGAs, not GPUs, so you should be fine.
(Yes, you can use it 24/7 with GPUs and it will work fine)