In regards to the size of the foliage, Phrag. sargentianum and particularly Phrag. longifolium are variable in size. It seems perfectly reasonable that if a compact variety or cultivar of P. longifolium was combined with a not so gigantic Phrag. sargentianum you would end up with something more manageable than the old school Phrag. Sorcerer's Apprentice plants that used the monster sized (and perhaps polyploid) versions of the parent species. Often you'll see vendors or breeders adding the specific variety of longifolium (ex: roezlii, gracile, etc) to their tags, but not always.