In this build, I've managed to get a first version unit selection working. Map selection works as before (but with less glitches):
The player hosting the game may select the type and number of units. Both sides get the same amount, and units are randomly assigned to each player on the team.
There's no hard limit on the number of players or the number of units. I'm not sure what's possible in practice without too much lag, so please go ahead and stress test this if you like. Please report your findings here.