Assuming that you keep the payload in the bin or near the top of the robot, 9 kg is a safe amount of max payload weight on a flat floor. Create doesn't dynamically balance, so having the weight in both a statically and a dynamically stable location is important. We've also provided files for printing a bin with a caster, in the case that you find your robot is constantly doing wheelies. :)
Almost all existing cables and accessories should be supported in both directions, as long as they use the 7-pin serial connector. One of our goals was to allow current parts and accessories to stay useful! Unfortunately, there is no 25-pin payload connector, so any accessories which used that will not work. Finally, the original Create's dock is not compatible. The good news is that the Create 2 will avoid its predecessor's dock, and we ship every Create 2 with a new-style dock.
Please note that I am an iRobot employee, but the postings on this site are my own and don't necessarily represent iRobot's positions, strategies, or opinions.