Custom games don't need 20 people to start the server. They only need one. If you play verified-mode, you have a 20 player limit, but you said custom. Custom has no minimum limit, unless you turn on bot-fill. The workaround is to never use bot-fill, and instead create AI spawners in the map and rules to spawn custom AI instead of using the built-in bot-fill.
Here are a dozen different experiences. All of them can be started with as few as 1 person, and bots are available. Unique Maps and Portal Love | EA Forums - 12909480
I have no idea what you think crossplay has to do with the ability to play custom games. Post unclear.