Both the slaves and the senate are playable, but it's important to note that they are much more buggy than the other factions. When playing as the senate, for example, the game crashes whenever you click on the senate tab in the faction summery scroll.
Basically, if you do choose to play either the rebels or the senate, be prepared for a few nasty surprises. Ensure that you save your game as frequently as possible in case of the occurrence of such random crashes.
BTW, as the rebels, it's frequently a good idea to disband forces, destroy and abandon settlements and centralise your empire around a single strategic settlement (usually in a rich area). Defending the scattered rebel empire is exceedingly difficult. Especially considering that the rebels typically are the favourite first target of the "real" factions.
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