Are you planning to mix the bale of promix with 2 bags of FFOF? If so I think you'll find that too light on nutrients for larger plants. It would be great for seed starting, but mature plants could easily handle FFOF on its own. Cut half and half with promix will have minimal nutrition and require supplemental feeding.
Depends what your plan is I guess. If it's plant and forget about it, you'd be better off in straight FFOF. If you're tending to it and feeding it here and there you'd be fine to mix the two.