To anwser your question on population, do you enough industry in your city, your demand should barly be positive. Plus on dense residential you need to altranate com. and res. or your commute times will be to long. But if you only have 6000 people in your city and you can't get it to grow you should probably start a different city next to it and come back later. You have to grow out before you can grow up. And their is a park pack that you can download in the exchange that will take care of all your pollution, but dont use the trash one because when you bulldoze it the trash will pull up uncontrollably. I dont think you even have to plug it in though.