Newbie Questions

I had rioting enemy surrendered planets in one game. The surrender came after I had blockaded two systems then moved the remaining ships in two fleets (both about 10 dreadnoughts) into the home and an adjacent system. At the surrender every planet was rioting and I just scheduled a build of UPC in each system and moved my fleets. Coincidentally the move resulted in over 20 dreadnoughts in the surrendered home system next turn. I got a message in politics Category that the population of that system had grown happier due to the military protection, and it had improved each planet in that system to angry. So I just consolidated the fleet and did the systems one by one. Before I had reached the Last system it had stopped rioting on its own, about eight turns. So you can try that if you can afford the fleet maintenance...
Definitely build UPC in every system. Another thing that helps is 150 large troops, which is a heck of a lot cheaper.