I've never used the EXOS sytem but, it looks as if it uses 3/8ID tubing. Without knowing just how powerful the pump is in that particular system you are basically governed by the same rules as in a DIY kit actually - your only limitation is the tubing diameter. If the pump in the EXOS system is relatively weak, though, you'll want to use a waterblock that is known for high-flow characteristics like the Apogee.
Mind you, I just intend to say that you don't "absolutely" have to use an EXOS waterblock, just be mindful of the diameter of the tubing involved (and the pumps capabilities, of course).