@Terminal I would be more alright with a small habitable world instead of a Medium. And also, with the situation you propose, and choosing not to fall under the rule of the Celestial Empire, yes, you would be Rebels. And being Rebels you would be sought out and destroyed, instead of being defended against, you would be hunted. And if you were able to construct a defensible fleet, by an internal decision, the Dreadnought may be used. When the roleplay starts, C.E. is going to be giving ARG Credits to Minors with small defensive capabilities to fund a military space station for added protection (And those they like). You'll have to quickly get allies if you don't want C.E. to come wipe you out when they find you.
Otherwise, you could be a collective of transporters, traders, and other civilians, giving you added bonuses, other disadvantages, still have a lot of credits, and not be seen as Rebels... until you become hostile.