Mobey Dick;
I own a Bucks Bag South Fork. I love the boat. I have used it on quite a few rivers, including some white water. The rivers you mentioned would suite this type of pontoon well. My boat is not too big or too little. The problem with large ones is maneuvering through tight boulders on smaller rivers, or canyon areas. My boat has welded seams and a stainless frame, the boat I think only weighs around 40lbs or so. I have packed it around a lot. I recommend if you purchase one, practice on some safe, slower water first, then move up from there. There is a lot to do with timing when leaning how to maneuver through white water or seam changes. I wear a helmet, a tight belt on waders, and a mustang life jacket collar which has an emergency inflater cord attached to a CO2 canister in the vest. I have done some nasty white water where the helmet saved my my friend. I flipped the boat and smacked my head on a boulder once, the bike helmet saved my my friend. Luckily I keep a teather rope attached to me and the boat with a quick release snap buckle. When I gained my composure I made it to shore as well as my boat. Some people recommend quick release ropes, others don't. A worker from Chehalis hatchery, Dana, used to run a white water boating business. He told me a story about a bad situation which can arise from the use of ropes attaching you to the boat. He told me that on one trip he was on in a large Zodiac, filled with people. The boat hit dead on into a large boulder in the middle of some hairy white water. Everyone was thrown out of the boat. The person who was wearing the rope got thrown on one side of the large rock and the boat went on the other. Unfortunately this rope did not have a quick release snap. The pressure of the water pushing the boat down one side and the person down the other quickly drowned and severed the persons body in half. Everyone else was OK, but they lost their boat for a while until another group retrieved it for them. Holy crap eh! Well, needless to say, I use a quick release snap when in dangerous areas. Even so, I am still a little weary about using it.
The main thing is to practice lots. I do quite a bit of white water, canyon fishing, so I'm used to that, but that is definitely the extreme end of things. The rivers you named are quite fine. The Vedders section which I would not recommend pontooning if you are just learning is Tamahi. Large boulders and folding "pillows" of white water make home in this section. The key is to know the area you are going to float through. When in doubt, get out and walk the boat down with a rope. I've had to do that plenty of times. The Thompson has lots of great pontoonable water, and also lots of ugly stuff. Another good pontoon trip, which Brian from West Coast tackle in Vancouver often does is the Fraser for Salmon fishing. He goes with some buddies, they park one vehicle up river, and then pontoon down to the other vehicle. They put coolers on the back cargo mesh and rod tubes attached to their pontoons. He said it's a great way to do it. I've yet to do this trip, but I can't wait to try it. I'm taking my boat up north this year to the Skeena area, I can't wait! I've done the trip with a Zodiac before but I think the pontoon will be much better.
Good luck, and happy paddling.