Can I use "any" handheld shower head on an existing Kohler shower ensemble?
There is a diverter on the main shower head "neck" and it sits on a slide bar.

Theoretically you should be able to use just any hand held showerhead replacement. If it doesn't fit, just take it back hoping you keep the receipt and original packaging. I know of the configuration you are speaking about and it probably has the same male end that every other shower pipe has.

You should be able to install just about any hand-held shower/hose on your existing Kohler set up. Just be careful: the hand-held usually rests in some kind of wall bracket when not in use. Make sure your new hose has one end that fits that bracket's shape / configuration. Otherwise, you will need to replace that bracket too. The incoming water end of your new hose is 1/2" IP and should fit w/o problem.