The rule of thumb for goldfish is 20 gallons for the 1st goldfish and 10 gallons for each additional goldfish, so 56 gallons is a nice comfortable fit for 4 adult goldfish.
As for compatibility, you should consider both size and type when it comes to goldfish. The slower fancy varieties don't usually mix well with the slimmer faster varieties like comets or shubunkins, but luckily orandas, ryukins and pearlscales are all fat and slow! (But very cute

) Goldfish are not an overly aggressive species of fish, so I think you should be fine with the size difference, especially since your tank is large. (Goldfish aggression is often a symptom of overcrowding)
If you're concerned about the smaller goldfish being picked on, you may want to have a divider or quarantine tank available to segregate the smaller goldfish if it comes to that. However, I think you should be just fine, and I hope you'll post a picture of your new fish once they're settled in!