About the casual playlist. I would strongly advice that it should be freelance. It's far from casual when a six stack "just having fun with friends" enter the lobby.
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I'd still suggest having fireteam based matchmaking to avoid that issue. Whilst it can still happen (6 stack vs 6 solo's) it's not a common occurrence, it's just a horror show when it does. I'm not a fan of freelance playlists as they split the player base into different pools which increases issue with wait times etc.
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The casual list could be freelance only and the competitive could be for teams.