The Manfrotto 234RC2 Monopod head will be OK with the 70-200. However, it will droop with the 50-500.
I really hate using a ball head with a monopod. I find I need one hand to hold the monopod, one hand to hold the camera and one hand to adjust the head.
You can screw the monopod directly onto the camera/lens, but that's a real pain to put on/take off. Many people use a clamp to fit the quick-release plates they have for their tripod. Both solutions are really only good if you shoot stuff in a single plane (like a football field). For anything in three dimensions (like birds in flight) it's a total no-go.
The best solution is one of - Really Right Stuff Monopod Head or Acratech Long Lens Head. Both are excellent, both are expensive.