I love speculation, and honestly both HL and 6vs6 have a lot of issues if they're going to be just thrown into MM.
HL, a lot of pubbers really don't want to be stuck to just 1 class, and trying to keep 18 people playing a full match could be a pain with a bunch of random people some who may/may not be fucking around.
6vs6, has a standard meta that isn't going to be immediately picked up on by pubbers, and if this is ranked (it likely will be) people at lower ranks will likely have the shittiest time imaginable with medics going engineer or heavy or some bs.
Plus 6vs6 is basically just 5CP with some KotH. I'm sure a lot of people would want to play PL and A/D
Would be interested in seeing 7vs7 or 8vs8 being picked up by Valve, but tbh I have no fucking clue.
-edit-
obviously certain classes would need limits of 1 like demo, medic, heavy and the rest probably capped to 2