I usually give conquered planets to a minor race (in exchange for tech if they have any). That way I don't have to manage them (and they don't screw up my carefully balanced economy).
I've found this enables the minors to better kick the tails of the major races and keep them off of me. I have yet to see a minor land transports and take a planet (meaning they don't go expansionist on me requiring war with them), so I don't see a down side.
If it's a PQ15-17 system, I will delete all social projects and gift it to them. That system will then act as a "speed-brake" on their Morale and Tax Rate, limiting their overall development pace until they spend enough bc's to "catch it up."
Works awfully well against the Drengin and the Yor. The Arceans and Altarians seem to have high enough Morales & incomes that they can absorb the new system relatively quickly. The Torians merely go into debt to "buy it up" to acceptable levels.