Glad to see you took the plunge. I'll still answer you're pre-sales questions here
Regarding JomSocial, yes, you'll need to disable their Facebook functionality to enable JFBConnect. They don't play well together. We don't have a way to insert the FB Login button into the JomSocial login automatically yet, but we'll be releasing a plugin to do so. If you want the button there, I'd suggest the following thread which will show you how:
www.sourcecoast.com/forums/topic?id=1227
Definitely let us know what you think. We'll also be releasing the final 3.0 stable version later this week. 3.0RC2 has been pretty rock-solid so far with only a few minor issues which will be fixed. Upgrading is always a cinch.
Thanks!