I'm sorry this is really a thyroid forum and we don't actually deal with gall bladder issues, however, I'll give this a simple answer.
Is it safe to assume that the lymph nodes involved in your thyroid cancer were removed with your thyroid?
Some types of cancer may be more likely to spread than others and the stage of cancer when it was found would also be a determining factor as to whether or not that cancer might have spread.
As for your gall bladder, I'm far from an expert on that issue, as my gall bladder was removed years ago, due to very large stones that kept moving around causing severe pain/attacks. The fear was that one would lodge permanently in the bile duct, which is a life threatening situation...
As far as I'm concerned, once a person has had cancer, any size polyp would be a concern and if my doctor wouldn't do anything about them I'd want a second opinion, especially, since yours are growing...I do know that with thyroid nodules, they must be a certain size (close to 1 cm) before they're large enough to do an FNA/biopsy on, so it's possible that your polyps are too small, at this point, to get good samples from. There are also certain characteristics that would produce more cause for concern and your polyps may not have any of those characteristics.
I'd suspect that if a cancer can spread, it would be impossible to keep it from going wherever it chooses to go. It's also possible that the thyroid cancer didn't spread, but a new cancer could start - not that I'm suggesting you have cancer, because I have no way of knowing that...
In my opinion, whenever one s in doubt about what their doctor says, they should always seek a second, or even third opinion, but we should make sure we have and understand the facts, as well. Our health is too precious not to take seriously anything that concerns us, but we have to very leery of "Dr Google" as well - I'm currently, in a situation, where I have to keep reminding myself of this, as I do my own research.
My question is - are you sure there are no more gall stones that could be rolling around in there causing the pain in your right side? I had right side pain for close to 20 yrs before they finally figured out that I had gall stones and I'd had a couple of ultrasounds that didn't pick them up...just food for thought.