I've found that I'll run into the 'data storage full' error if I add any additional google calendars under the same google account, but that deleting the caldav account on the n9 and then adding it again will pick up the new calendars.

also, make sure you save your changes on the iphoneselect page.

for calendars shared/subscribed from some other person, it's also important how they are sharing it, and sometimes it's not possible to get the calendar data, even though you have a caldav URL for it. i ran into this problem with a work-related google calendar when they gave me the rather long caldav URL and I tried to add it manually, but i was able to access the calendar in question under my properly authenticated caldav account on the n9, if that makes sense.

there are also some common google calendars that can't be added with caldav URLs, e.g. some of the holiday calendars, and the solution is generally to find a publicly available .ics file of the calendar you want, edit the .ics to taste, and then import the data into an existing calendar (or create a new one).