One thing done in 7DS for TEC, the corpo-nations that have pulled away from Trade Order, is they have an Outreach module that hires mercs from the pirates, advent remnant, taalo, TCA (military arm of Trade Order).  Each costs some credits, metal and crystal, but allow the TEC to spawn (return armada style) ships of the above factions to the planet to assist. 

Something along these lines for rebels would permit them to attack pirates and rebels in their way and avoid the massive idiot build up that happens now, but beacon in pirate forces from 'outside' who have no loyalty to the pirates/rebels present on the map.

http://pastebin.com/dwp6eJns

Combine with a raider xenophobia that reduces pirate strength against the rebel TEC faction and it's great.