If you haven't aluminium in loop (and as you mentioned - all your components are nickel/copper/brass/stainless steel), any coolant will do, as you don't need to care about corrosion. What i can advise - distilled with something to prevent algae growth. For example - killcoil or few drops of some biocide (eg. PHN copper). Glycol sounds like some antifreeze? No need for that (see above). It will just worsen thermal transfer properties of coolant.