Get a pond, man. Shubunkins = Koi, AKA 3+ feet, and common goldies get almost 2 feet. If you take them back, take them to a LFS, not the !@#$% holes at WalMart!
While I agree that a pond is best for Common's, comets, shubunkins, etc... any long-bodied goldfish, I just wanted to clarify that shubunkins are not Koi. Shubunkins are merely calico common type goldfish. Shubunkins usually should grow to the 12"-15" range where pure common goldfish can grow to nearly 24".
I'm glad you are getting a 55G but realize that even it will not be large enough for the four or even one long-bodied goldfish as a permanent home. Work on finding a nice little pond for them so they each have a 100G of water volume and lots of swimming room. You will probably have to do weekly to twice weekly PWC's (25% partial water changes) on the 55G and constantly run chemical filtration (carbon changed every few weeks or Purigen or other advanced chemical filtration that can be cleaned as needed) to help with removal of hormone levels, etc.
Think about buying some Anacharis/Elodea and just leave it floating on the surface. It will get more light and CO2 from the air and grow faster so it will also help with sucking up the nitrogenous waste of the goldfish and increase the CO2 levels as well. Also, by letting it float, instead of planting it, it makes it harder for the goldfish to eat. They'll still pick at it but at least they can't rip it to shreds as quickly. You can normally get Anacharis cheap for around 50 cents a bundle at garden centers and some LFS.