I think that you will find the 80Hz upper end will be satisfactory for all of the athletes you use it with. I’ve tried it on several fast twitch dominant athletes (i.e. vertical jump of 40+ inches) and it always seems to pick up the muscle pretty good.
Number Two,
In general all muscles will be picked up at any frequency one selects. Because the stimulation is involuntary, all fibers are involuntarily recruited. Even at 20 or 30 Hz, you will be able to recruit 100% of fibers if the current intensity (milli-Amps) is strong enough to penetrate deeply enough in the muscle bundle: slow twitch type I, fast twitch type IIa, and fast twitch type IIb will all contract. In my opinion, with EMS we are trying to squeeze out a little extra force, i.e. little extra recruitment. Experimental recruitment curves show that at 80Hz muscle force and recruitment have not peaked yet. The trick I think is to work at the frequency closest to the frequency at which the fiber to train would naturally peak. If this line of reasoning is correct, to tweak explosive force fibers it may be better to work above 80 Hz, that is between 100 Hz and 120 Hz; these are at least the frequencies utilized for explosive force programs found in Globus and Compex machines. My reasoning is based on the theory of muscle electrophysiology I’ve learned from various research papers, but again I do not have your S&C background and experience and you may prove me correct in theory and incorrect in practice. 
Graph from:
Enoka, Roger. 2008. Neuromechanics of human movement. 4th ed. Champaign IL [u.a.]: Human Kinetics.
I was under the impression that microcurrent therapy device would cost more
what you guys think of this unit?