They could share supply...
This probably works best (if you could find how to remove races (seen it done)):
QUOTE(Voyager7456(MM))
The closest thing I've come up with is to have player start off with a building and be able to morph it into 4 different Town Halls.
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But if you have protoss w/ mind control(I know it doesn't happen much), then you'd have to split the supply. Plus, how'd you get them into the game? Only UMS would work that way.
But, That town hall idea is interesting.