I understand your worry, but you'll be lucky to hit 1Gb on cloud data. Watching a 1080p stream is a lot different to a couple of compressed bits and bobs.
If you move to a different console on the PS4, will you be able to download your game saves or anything?
On basic cloud data, it's not much. However if games are installed to/saved on the cloud. Then the transfer of that data, and the caching of resources and such will eat up internet fast.
Also, as far as I know.. yes you will be able to get game saves and such. Plus.. I don't mind if I use up someone elses internet (if they are ok with it). but having as many of us in the house as we do.. the internet usage has to be monitored very closely. The past 2 months (due to kids sneaking onto netflix) we have been charged an extra 60$ (45gb) in overages.