1. The only way to decrease plurality is to time your research of Ideological Thought to a tee. Ideological Thought will trigger a chain of inventions which will either decrease your plurality (five -5 plurality inventions in total) if you have a conservative/reactionary party in power, or increase it (five +5 for socialist/communist and five +5 for liberal/anarcho-liberal) if you have a socialist, communist, liberal or anaracho liberal party in power.
Plurality is good for democracies, does not have any effect on constitutional monarchies, and is bad for monarchies.
2. Depends entirely on the way you want to play the game. If you like the party you have in power at the start, then a monarchy. A constitutional monarchy will appease the people somewhat, while allowing you some control. Democracies are good if you know how to handle them, and they are great for immigration.
There is necessity for a monarchy at the beginning, but after a certain stage, I prefer democracies.
3. By either buying or selling the claims to the other nation OR building one of each type of colonial building in the colony (all 4 for a coastal colony, and 3 not including a coaling port for an inland colony). If there is an inland colony which has 5 provinces, and the AI nation builds to colonies there, but you are able to build 3, then if you build a fort, a mission and a trading post in those 3 colonies, you will be able to claim that colony. However, in order to claim a colony like this, all provinces need to have colonial buildings built.
4. There is some invention as a result of a cultural tech that will give you 3 random claims, most likely somewhere along your border, depending upon what party you have in power. I think you need to have a jingo/pro-military party in power. I cannot remember the name of the tech/invention though.
