HydrX has dyes and other such things that can break down with temperature(just search google for pictures of waterblocks getting dye gunked into them, it happens not only with HydrX but dye bombs, feser cooling fluids, premixes like pc ice, etc). It will take time but it will eventually cause gunking of blocks...and remember, its only a corrosion inhibitor, it does not fully prevent galvanic corrosion(especially on that cpu block i'd watch, because you have aluminum and copper in direct contact). Using HydrX is not ideal, but the only time i would suggest running it is in a case such as yours with mixed metals.
Yeah I really just bought this block because it was cheap, and socket 478 blocks are limited. I wish I'd have seen the one you linked too, I would have definitely bought that instead. Oh well.
This isn't a long term build, it's only a ''fun rig'', I doubt overall the coolant will see 48 total hours of use (couple hours here and there) before I get bored with it and do something else.