Eddie, I can answer a couple of your questions.
1. You should only run the exact same version of nodes within a Sense site. The server communication API may change between versions and you could end up with a corrupted node if they are different.
2. Within a site (a collection of nodes), there should only be one QMC. There can be as many hubs as you want and you can configure access to those hubs.
For testing/upgrade purposes I would set up a separate TEST site with its own set of nodes and separate QMC.