Well we've been using newborn clothes mostly, but she fits better in preemie (she was 4lb 12oz 3 days ago).
They are not too bad so far, mostly because Lisa is really easy. She cries when she's hungry but that's it. Chris complains more because of gas, or because his diaper is dirty etc... I'm not holding my breath, they will be one month in 2 days but technically they should not even be born yet so I know the worst is to come, lol! It really suck when they cry at the same time though... My husband goes back to work tomorrow and I'm dreading it for that reason. That, and I'm probably going to have no time whatsoever for myself
For diaper rash, she used to have a bad one with pampers... we switched to luvs and now no problem at all (although I hate their smell). You might want to try a different brand of diapers maybe?