I'm a big fan of monopods for the right situation. They can be challenging for subjects high above your head, but for subjects in the 8-11 o'clock elevation it helps a lot. My preferred lens with a monopod is the 200-400 - about the same size as your 500 f/4. I've used it with my 600 f/4 but prefer a gimbal and tripod with that combination. I also use a monopod with my 300 f/4 and teleconverter. If I'm photographing small subjects that need me to stay on them for extended periods of time - such as small birds or insects - a monopod helps a lot so I even use it with a macro lens.
I don't use a gimbal - it does not work well with good monopod technique. I use a RRS monopod head which is a tilt head and keeps movement on one axis. To elevate the view, I use the tilt head or simply tilt the entire monopod. To swing left or right, I use my feet. My monopod is a 4 series Gitzo. You want a robust monopod - 4 series or equivalent. It does not weigh much more than a smaller monopod, and you want something that handles a long lens.
The key with a monopod is having the right technique. You don't want to use the monopod in a vertical position - it works better at an angle similar to the angled legs of your tripod. There are other options for monopod technique where you use a tree for support, or where you brace the monopod against your rear foot. It's okay to balance a monopod vertically, but it's inherently less stable than angled alternatives. Think about your monopod as a tripod with only one or two legs - and you need to supply the other legs to create support.
