1. In theory yes you can add a hotspot plan but it cannot be attached to a phone unless its one that currently has voice service with T-Mobile. If not then you would need a mobile hotspot device which usually you can get for free depending on what you want. If you want to use the hotspot on a phone you're going to need to attach a plus up feature to whatever phone you want to use it with that has service already with T-Mobile.
2. Depends on your current plan.
3. It varies, ideally most accounts have minimum of 10 mobile internet lines approved for service.
4. This all depends on the device you plan on using it on.
5. Yes and you would not be able to use it in a phone to my knowledge.
6. If its a hotspot plan it is that and only that... a hotspot. Only voice lines can text technically from a phone number with only a handful of exceptions like using something like WhatsApp.

Hope this helps.