question

PlayFab core service, along with Multiplayer Server or Party are designed to serve different purposes.

• PlayFab core service

is a service based on RESTful API, few requirements you have mentioned can be achieved without multiplayer server and Party.

• PlayFab Party

Azure PlayFab Party is a low-latency chat and data communication solution for cross-platform and cross-device multiplayer games. The voice and text features in Party can be used as a standalone chat solution.

• Multiplayer Server

For multiplayer games that use a server-client authoritative architecture to curtail competitive cheating concerns or support devices with limited resources, consider multiplayer servers like PlayFab multiplayer servers to host custom, dedicated game logic in a dynamically scaling cloud environment and use Party for chat communication.

>>Matches of 4 players or 8 max.

Matchmaking system and Multiplayer Server is separate and independent. The Matchmaking system will match players via specific rules you have defined and only when “auto-allocation” is enabled, will the multiplayer server involve. Simply set the Min as 4 and Max as 8 in Match Size will meet your requirement. Please refer to -- Matchmaking quickstart - PlayFab | Microsoft Docs

>> Competitive matches & rankings

The matches you mentioned should be real-time, which means multiplayer server should be involved. You are free to develop your own server build as long as integrated with PlayFab GSDK, which help PlayFab server manage the server instance. Please refer to PlayFab Multiplayer documentation - PlayFab | Microsoft Docs. In terms of ranking, PlayFab statistics&Leaderboard can handle it --Tournaments & Leaderboards - PlayFab | Microsoft Docs

>> Constant LiveOps for new updates

Surly supported. PlayFab is designed for developers to run the game as a service with telemetry, automation, content management, and in-game commerce

>>Cross-Platform Support

PlayFab provide SDKs for most popular platform and due to nature of RESTful, basically there is no limitation on the platforms as long as the HTTP request is permitted. Players are able to link multiple login identities and many 3-rd party services are also supported. Please refer to Platform-Specific Authentication - PlayFab | Microsoft Docs.

>>Custom/Private Matches

May I ask does it means customized/private games that a player can host and invite other players? If it is the case, then yes, it is supported as the server build is defined on your own. If it means customized matchmaking, it is also supported. Through configuring Rules, you can let the matchmaking algorithm determine which tickets should match together. May I know more details if it is private matchmaking?

>>Group creation for playing with friends

Yes, it is supported. Please check Groups, Guilds and Clans - PlayFab | Microsoft Docs

>>Steam integration

Yes, it is supported. There is also an official integration tutorial on Setting up PlayFab authentication using Steam and Unity - PlayFab | Microsoft Docs