Well I think with Jerry/Newman, to me anyway, it was explained by what was going on throughout the series. Newman is a little bit jealous of Jerry (has friends, has a pretty nice career (Id much rather be a comedian on the road going to different places than deliver mail) and also we find out later on hes developed a crush on Elaine so he may be jealous of Jerry for spending so much time with her. Newman on the other hand is just plain annoying and kind of weird so I think he rubs Jerry the wrong way (plus with how Newman is about Jerry, Im sure he doesnt exactly treat him like gold). So eventually over time they just started to resent eachother.
However, we never do find out about Kramer but I think thats one of those things about they show we not SUPPOSED to know. Kindof like on shows how they reference people and we never see them? (Maris on Frasier is one I can think of off the top of my head). I just believer Kramer was retired and did LITTLE things like work at the Bagel Shop, Kramamerica, etc to get a little bit of money here and there. But for the most part thats how he has ANY money. Everything else were not "supposed" to know.