One thing I don’t understand for the matchmaking on Reach for the MCC is that heavies is placed under “Action Sack”. Given how there really isn’t any special rules about the game mode just extra vehicles, it should just be included along with regular slayer in matchmaking. For 8v8 action sack you have elite slayer (which I believe is the worst gametype), which changes your loadout, and buddy slayer where you control where you can spawn which seem appropriate for action sack. However heavies doesn’t really change that much about the game, as stated before its just extra vehicles and the score count is increased to 150.
TL;DR - Add heavy slayer to regular slayer for 8v8 in Reach for MCC.