Not Krosovs, Ogrovs. (Or Orgrovs??? Not quite sure on the spelling) Ogrovs can only attack buildings. Krosovs? Don't waste your time on these, they are not worth the supply cost, for only a bombing use.
Also, do not bring in Ogrovs until th pirate fleet is gone, or they will die. Without fail.
And don't forget to colonize the pirate base. Once you research the TEC's aditional logistics slots, they can accomodate a factory, which will usually become your main shipyard since pirates tend to be near the center of the map.