I have a big butt too and I think it helps flatwater cartwheels!
Flatwater cartwheels require more strength than cartwheels in a hole because you don't have any current to help you out. If you're getting sore triceps it's probably because you're trying to muscle it with your arms and not use your torso. Torso rotation IS THE most important concept for spins and cartwheels. Your upper body shouldn't move back and forth hardly at all, but should be rotating. The best way to practice this is with the double pump. Try taking baby steps with the double pump (even if you already know it). Start by putting your boat on edge DO NOT use your paddler at first and just rotate your body between your knee that is dropeed and the bow edge of your boat that is dropped (so you always stay to one side and don't rotate all the way over to the other side of your boat). Your rotations should be quick and short. You'll notice that your bow will dip in and out of the water. When you get good at that part you can add your paddle. Plant your paddle, but DO NOT paddle with it. Pretend like you've just planted it in cement and you focus on the rotation of your core instead of trying to move your paddle back and forth in the water. This takes practice and mental training!
Once you have your torso rotation dialed it'll be a lot easier to flatwater cartwheel because you'll be using big muscles. Remember that for initating in flatwater your 'pull' stroke is much more important that your push stroke. Everyone thinks that it's the push stoke that gets your bow down, but it's not true. Your pull stroke is what lifts your bow up which creates momentum and allows you to put your boat on edge. Remember to push down on your feet and pretend like you're doing a sit up once you are doing that push stroke. Also, make sure you're looking ahead and not down at the water on the side you're trying to initiate. All of this is much easier to teach in person!
You don't need to flatwater cartwheel in order to cartwheel in a hole, but you DO need good spin technique. Spinning is something that people tend to overlook, but if you can't spin well in a hole (at least three times completely in control and knowing where you're looking) then you can't cartwheel in a hole either. So spend some time on your spins.
The size of your boat does matter for cartwheels. The bigger your boat is the harder it is to cartwheel. If you can, try to get in a boat that's too small for you and really slicey (like a Kingpin 6.1) for a pool session and play around with it. You can even get in a squirt boat. This way you'll get the feeling for what you want to do. Putting water in your boat is another option.
I hope that answers your questions!