About low pocket pairs, limping sounds a good decision, but it really depends how the others have played the hand until you. I don't like to limp, but in cases when there are a lot of limpers, if I am with low pocket pair I will limp too, because if I hit a set on the flop, someone will hit something else, too from the flop because of the lot of limpers and it is a fantastic way to steal their chips. I love to play low pocket pairs, because the decisions are not difficult. If you hit something on the flop, you know that your hand is very strong. If you don't hit it in most situations is an easy fold.
About middle pocket pairs, it is good the make a good raise and to see after that what will happen. But again, it really depends on the blinds, players stack and your current stack at the moment. If your stack is solid, you can start with good raise and if many people have called you, it is best to check on the flop if you do not hit a set or there are a high cards, straight draws,flush draws on the flop. Sometimes, it is the best to go all-in with middle pocket pairs.