What are the dimensions for the 33?
Commons can get pretty big and while the 33 would be good for a while for bio-load Commons like to swim alot. I have my fancy's in a 56 and they use the whole tank in their explorations.... A common would swim even more...
What are you considering a common? Like are you talking about comets (feeders) or fancys or what I'm not sure I know what a common goldfish is. If it's a comet then your tank will be too small for even one. It'd be fine for while its small but you will stunt it's growth unless you move it to a massive tank or a pond. But let me know about what you mean by a common goldfish
Commons and Comets are basically the same thing, the only real difference is that Comets generally have longer fins.
That said, I would not recommend either for a 33 gallon for it's whole life. They can grow up to two feet long. If you want to know if one will fit in your tank, take a couple of rulers and tape them together end to end so they make a 2 foot long stick. Float and swim it around the tank. If it bumps into the walls or the decorations or anything, then you know a fish will have the same problems. If you really want a goldfish in that tank, it would be okay for 1 or 2 fancy goldfish. They only grow up to about 1 foot.