Quality "dry" foods do not cause digestion issues, and yes I have kept fancy goldfish over the years too.
They ate the same food as everyone else in the fish room. I also discussed the age old fable of pellets causing bloat in more detail in the following past thread, that was later made a sticky.
https://www.monsterfishkeepers.com/forums/threads/bloat-causes-cures-and-big-myths.456034/
The gastric acids and enzymes found in a fishes gut turn dry food into soft mush in very short order. In fact, I have seen steel hooks dissolved over time by these same gastric acids. Having said that, if fed too much dry food, too quickly (overfeeding), or if fed a dry food that is high in lower quality fillers, those two conditions can cause issues in the digestion department. But you can't blame either of those conditions on the food being dry. The first issue is caused from operator error, the second from lower quality/less digestible ingredients. I discuss both of those scenarios in the link above.
I also discuss why one should proceed slowly when suddenly changing a fishes diet.
If the poop looks like a bunch of little pellets stuck together, it's possible that's exactly what it is - caused from overfeeding, and a sudden change in diet. You just bought these fish from a LFS, which I think is safe to assume were feeding a lower cost bulk feed at the store. Ease up on the food and allow the fish to become acclimatized to their new surroundings, and their new diet. These are probably pretty stressful times for your new fish.
Good luck
