Reasearch: Housing or Agriculture or both
Collective have high production rate for agriculture. Collective have basically no per turn income. Collective make money from plundering planets. High pop on a planet means more money from plundering it. A basis for Collective to make money is to reasearch both housing and agriculture, build agriculture to make food for future use while your pop is growing. Build pure housing on planets and clusters to get the most from plundering it. When your pop is gowing this is where your food reserves you made will come in handy.
Note: Collective cannot use U Class planets cause they cannot assimilate them. For a collective to be able to use a planet they must assimilate it.
Collective assimilate planets so they can use and cluster them. If they assimilate a cluster they capture it becomes a similar cluster and cannot be clustered futher.
Note: Collective are a warrior race and cannot explore. They must attack and fight for their planets.