I would siphon out as many as possible prior to treating. Then do the treatment, if you don't see any dying within the first 15 mins or so I would add more. I never have much success with the dosage on the package so I usually go to town with the stuff. I siphon the flatworms out as they die, they form mucousy strings and sort of float. Let it go for about 45-60 mins or so, then run carbon and do a waterchange. I don't know if either of that is required but I'd rather the stuff not stick around in my tank.
I don't recall ever treating my large tank, just my nano which I can't remember if it had fish in it at the time, make sure you have a few powerheads pointed at the surface for aeration just in case.
Other than that I think you're pretty much set!
Oh and +1 on the wrasse, I had a melanurus wrasse that kept the tank flatworm free but some of the yellow coris wrasses and other wrasses have been reported to eat them, might save you from having to treat the tank again.
