Is this what I would use alone for adhering 10 lb. rock veneers around my fireplace? Does it come in smaller bags? I can't handle a bag this heavy!

No, QUIKRETE 94 lb. Portland Cement is intended to be used blended with concrete sand and gravel to make a concrete, with hydrated lime and mason sand to make a masonry mortar, or with hydrated lime and plaster sand to make a stucco. A pre-blended Type S masonry mortar, such as QUIKRETE ProFinish Blended Mason Mix, or QUIKRETE Veneer Stone Mortar, both which come in 80 lb bags (vs the 94 pound bag of the Portland Cement), would be suggested for this application.

No. You want to use a thin set mortar. You can actually just find a thin set mortar that is int he floor tile section of the store. you want a pre-mixed mortar, because it will not have any stones in it that will get in the way of your resettign the stones. (This "Portland Cement" is the active cement ingredient in concrete/mortar products and a 94# bag will make much, much more than you need.