There is no family data plan at this time. The $30/unlimited data plan is tied to the phone line. In my case we are going from 3 cell phones with 700 min and unlimited text messaging. In our case we are adding 3 Iphone and my bill is going to go up $90 ($30 for each iphone data plan).
As for the second quesiton, there is alot of confusion about the supposed 5GB cap. The 5GB cap that everyone refers to is for the mobile broadboad cards/usb modems. There is not 5GB cap for smartphones.
I hope this helps.