you only need one if your coolant does not have stuff already in it to kill the nasties.
mayhems premixes (aurora/x1/pastel) dont require a coil.
essentially yes, although anti-corrosive additive isnt a bad idea
with biocide you need to make sure you add the right amount as it can make the coolant acidic iirc.
I personally use x1/pastel because im lazy