So when you beat a player that has a rating higher than you, you'll gain more points than when you beat a player who has a rating lower than you. When you lose to someone, you lose more points if they have a lower rating than you, and less points if they have a higher rating. Does this mean that when joining a rated game, it's best to have the lowest rating? If you lose, you won't lose many points, but if you win, you will gain a lot. Should I be actively seeking out games with high rated players, and avoiding ones with low rated players?

That's one strategy, but remember that players with higher rating mean that they are better, so it's high risk, high reward: do you want to join a game with lots of good players and get lots of ranking by winning, but also lose a lot, or join a game with same/lower rated players and have more wins that are worth less?