For the record, it is already possible to challenge other users with any gen8 ffa format, thanks to the /challenge command. For example, if you challenge an user with the command:
Code:
/challenge gen8freeforall @@@ +all pokemon, gen8nationaldexag, !obtainable
You should be able to get something quite close to custom games (though this might not be the best command, i think it still has some ubers banned for some reason).
An example that might be easier to understand is applying, for example, stabmons rules to ffa challenges:
Code:
/challenge gen8freeforall @@@ STABmons Move Legality
If you type /battlerules anywhere on PS, you will get more detailed explanations on how this command work, and how you can customize its rules. Hope this help!