I messed around a little bit in single player and easily took down the Occupation Planet with my titan, which seems a little anti-climactic. I think it should be harder to take it down. I recommend upgrading the Vasari starbase and maybe adding a couple more of those special occupation capital ships.
After I colonized the planet, nothing really happened. I thought there would be a countdown timer like for the research victory option. Do you have to max out the Occupation Planet's health and population for an occupation victory?