You're free to use JFBConnect on any of your domains and you don't need to buy a new subscription for additional domains. If you are a developer and building sites for clients, we do ask that your clients each purchase a subscription. I don't think that's your case though, but I could be wrong.
JFBConnect v6, JomSocial 3.x and K2 should all play very nicely together. We test that combination regularly and many users are on the same configuration.
I hope that helps explain, but if you have any other questions, just let me know.
Thanks,
Alex