Activating iPad 3 Cellular Connection

Background:

I have an AT&T iPhone account, and my landline is also AT&T (both local & long distance); in addition I have combined billing of both those accounts. I was wondering if anyone here has the same situation and if they could answer a few questions.

I'm going to be staying in my mom's house while some electrical upgrade work is being done on it, but she doesn't have any Internet connectivity there so I'll be relying on the cellular network for checking my email. While I could do it using my iPhone's unlimited data plan, the small size makes typing on it difficult, thus I'm considering activating the iPad's cellular connection. My questions are:

Is it better to do so in an AT&T store or online?

Since I assume the iPad account will be separate from the iPhone account, should that bill be added to my combined billing or is it better to leave it separate?

If it is combined is it easy to de-activate/re-activate?

Thanks for any help you can give me.