Bootstrap is used by our current default page, but that doesn't mean it's a part of Fuel in any way, shape or form. We'll probably remove it for the next version just to prevent this kind of misunderstanding.
FuelPHP is a PHP backend framework, we make no assumptions or recommendations about what CSS (framework/reset/...), (X)HTML(1.0/1.1/5.0) STRICT/TRANSITIONAL, or Javascript (framework) you use on the frontend. That's completely up to you and none of our business.
Not on "Oil's admin generation", but when you generate forms and buttons and all that......... maybe it could be added as an extension like www.cniska.net/yii-bootstrap/