so if this bag has an underside sleeve for the pad, is it required that you use a pad since the underside insulation has been eliminated for this purpose? do i have to use a hard to find or special smaller sized pad if i order the petite?

Because Big agnes makes the sleeve there is no insulation on the bottom side. You do need a sleeping pad, but they are not hard to find. Just as long as it is a Big agnes pad it will work fine and many come in petite actually.
Take note of the shape - "squared off mummy" make sure the pad is that shape and a petite and you are rocking!

http://www.backcountry.com/store/subcat/17/Sleeping-Pads.html#app=price%7E+group%7E+brand%7E100000012+sortby%7E+cat%7E3+subcat%7E17+offset%7E+on_sale%7E+page_name%7E

That is a link to the big agnes pads.
View Details: Big Agnes Ethel Sleeping Bag: 0 Degree Down - Women's >