They love bloodworms by the way. Too much flake will bore them and make them constipated.
Actually it is somewhat the opposite.
Goldfish are mainly herbivores. A diet to high in animal protein can cause them to get bloat.
A good vegetable flake or spirulina flake coupled with say bloodworms several times a week will be a very good diet for them. It is also a good idea to presoak the flakes in some tank water prior to feeding.
Goldfish are voracious eaters and will very often come to the surface to eat. This in turn causes them to take in excessive amounts of air into their digestive tract.
By pre-soaking the flake it will sink almost instantly enabling the fish to eat it from the water column instead of the surface.
If you decide to feed a pellet food (this will create a lot less mess) you should most definitely pre-soak the pellets. Pellets if eaten in large amounts quickly (by any fish, not just goldfish) while dry, will swell up in the digestive tract and this is what causes constipation!!
You can also include fresh veggies such as peas (without the shells). This will help keep the regular as well.
Keep the water temp lower than for regular tropicals and plenty of filtration as goldfish are indeed messy (almost as messy as oscars)
HTH.
CM