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That sounds like a very reasonable architecture, though you should also be making sure to check the response info to ensure that if the session ticket has expired, you can re-log the player back in (so, if your login scene is the only place that manages sign-in, this may need to be added as part of your error handler in the lobby scene's managers). We do have a MOBA game sample you could also have a look at, to see one example architecture, here: https://github.com/PlayFab/UNION-OpenSource-MOBA.

Matchmaking in our service is for use with custom game servers. So if you've written a game server that your players will connect to for gameplay, and you want to have it hosted in our service, you would use the Matchmake call on the client to connect players. If that's what you need, and you specify a StatisticName, our matchmaker will automatically look for an available session where the other player(s) in it have a score as close as possible. We don't have a built-in ELO system, but if you were to calculate and store ELO values as statistics, you could use this in that manner. If you require more precision on the matchmaking or different logic, you can also plug you own matchmaker into our service via the Matchmaker API. That allows you to have your own matchmaking logic, while still using our service to host your game servers.

If you're using Photon Cloud, you would need to use their matchmaker logic to get players connected to Lobbies/Rooms. There's a write-up of this in their site, here: http://doc.photonengine.com/en/realtime/current/reference/matchmaking-and-lobby.