If you were to start a hangout via Gmail from your Google Apps account, you get a dumbed down version that doesn't support features like screensharing and up to 15 people.
You would need to use an account that has G+ connected and start the hangout from the hangouts app or from G+. So I think yes, you would need a G+ account to use this unless Google changes that.
Well sure. But when you start using services that require some kind of G+ opt-in then all of a sudden my Google account has a presence in Google Plus land - which is something I may not want my employees to have associated with their Google Apps accounts.
