Minors/majors joining me

No you don't but you get what's in their build ques. So if the the guys got a dreadnaught in the build que 2 months from completion, you get it if you complete it, even if you don't have dreadnaught tech yet. You can even end up with 2 manufactoring capitals I believe.

ED brought up an interesting point. When you take over a planet (or one defects to you), make sure you check the things they are building AND, more importantly, click on the details tab to see what's already been built on that planet. Scan the list of things already built on the planet, the number in parenthesis is the cost in BC per month to maintain this improvement. You can select the items you want to remove (such as the high cost things that you don't "need"). This will help you more effectively manage the expenditures/economy of your empire.

One other thing, majors will surrender and join you but I have never seen a minor surrender.

When the major surrendered to me it was their last planet and my BC per turn shot up by 300BC per turn to 560BC per turn cool eh.