| >are they limited to the noun sack, or they limited to wear they may be worn?|
They don't have to remain a sack, as long as the alterer is willing to adjust it. We generally don't adjust the location things are worn at via alterations, but again -- since these aren't normal containers and only serve a single purpose, it's up to the alterer if they can justify adjusting the straps or whatever may be required.
>Is there a chance that the tier 3 kittens will interact with summoned shadowlings?
Pretty sure shadowlings fall under the 'npc' category; they should be a target for interaction depending on the kitten's mood.