My spouse and I use a common Apple ID so that we both have access to all of the apps and music we have purchased. We also maintain completely separate e-mail, calendars, task lists, and so forth. We've never had any trouble with it. Every once in a while, if one of us gets a new app, it will automatically show up on the other one's iPad. If it's something I don't want, I just delete it from my iPad. That doesn't affect his set up at all. It does mean, though, that we always know what the other one has downloaded. That's not a problem for us, but if you want more privacy from your parents, that could be an issue I suppose.
By the way, we use Google calendar and e-mail. They work seamlessly with the native calendar and e-mail apps. For some odd reason, Google tasks won't sync with the reminders app on the iPad. No problem there, we just use the free GoTasks app.