I can imagine 4 on a shadowcast 16 can be pretty sketchy. Like Backcountry mentioned, it can be a tippy boat. I have a Shadowcast 16 and in my opinion, its meant for 2 adults comfortably or 2 adults and a child is the limit for that boat. Ive had 4 adults on there before and it was pretty dicey. If anyone were to move or shift weight, everyone on board would have to hold on an shift weight in order to compensate. Not only would it be tippy, there would be very little space for anything else. Even with two people on board, if one person were to walk or move somewhere, you have to let other know what youre going to do so they dont fall out or slip. Peaking at the CG decal in my boat, it says its rated for 3 person/ 425lbs. I would agree with that rating. Mind you, this is all personal opinion based on my experiences and im sure other members will chime in with different opinions. With that being said, I wish you the best of luck on your search for the right skiff.
In regards to the gheenoe, i had a classis 15'6 but that night and day difference from the LT class. I have no experience with an Lt25 so i wouldnt know how it handles multiple passengers.