The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
Resources are kinda unorganized so I'll point you to what is needed. It's a relatively simple mod:
set up the mod_info.lua (modding intro)
add the mod's lobby options file (guide).
Make your new lobby option with the same key as the default lobby option, which is UnitCap (source).