The kit you linked uses the windshield washer fluid tank as the source, no additional tank is needed. If you want to run a trunk mounted tank, that's fine too; To monitor tank level you'll need the sensor for it, something like this http://www.coolingmist.com/pagedispl...ure_key=safety
It's available from many different sources, and can be wired to either a warning light and/or into a standalone progressive controller.
Adding it and running the standard st.2+ map is fine, since that map was designed to be run on just pump gas. You probably won't see a significant bump in performance but it'll be easier on the engine due to the cooling properties of the water/meth.
The WW kit in itself is safe, the warning is really for how aggressive of a tune you run; For more aggressive tunes with a significant meth concentration, it's also treating the injection as a fuel supplement and runs more boost and timing than what is possible on pump gas alone. Should the meth run out or the system fail for any reason, with a flash tune there isn't safety integration to prevent engine damage. That's where the piggyback tunes shine in that a flow sensor can be wired to them; they'll automatically scale back boost and timing to safe levels if there is a lack of flow.
Our cars are already equipped with an oil cooler, and a supplemental water cooler. No point in removing them.