Is there quick matchmaking in Shogun 2

The advantage is actually not that great. If you play a lot you do get more retainers and more experience for your general but it's overall strategy and tactics that will win you the game. My general currently has 8 stars and I remember when I started off I beat a 7 star general without using any veteran units or retainers.

As to how the players are matched, if you let the game manage it, it will try to find a player of your level if one is available, if not, it will keep searching for any available player of sufficiently close rank.

When you achieve enough fame, the shogun considers you a threat and orders all other clans to declare war. This includes allies who you may have had since the start of the game.

The mechanism by which this work is that all your relationships are set at -40 and continue to decrease as turns pass. Now, any new vassels you create will not be affected by this penalty (nor will their relationships).