Better to not mix brands on one machine, the stability suffers quite a bit.

 Also, Vega is only GOOD at Monero mining, it's pretty easily beat on a $/hash or $/watt or often BOTH basis on everything else I've seen figures for it (D.Net RC5-72 work being the ONLY other exception, but that's not really cryptocoin mining related).

 My single Vega 56 doesn't run particularly hot, even though it's pushed a bit PAST it's "default TDP" currently.
 Can't speak to the 64 as I don't have any of those.

 Might want to consider waiting for non-reference VEGA designs to start showing up (in theory later this month per some "prelimanary" announcements but no solid announcements yet that I've seen) if you're worried about cooling.