They don't need to be able to shift, they should be bolted tight to one another but they can work loose and move around. That being said they are different metals and can expand and contract at different rates but that should not cause the problem you are having. "popping the paint out" Is more than likely poor preparation, cheap primer, bondo coming loose, etc... If bodywork and metal prep are done right and with quality products then you won't have these issues.