Koi and goldfish will eat any fish which is small enough to fit in their mouths, and considering how large goldfish and koi can get, this puts a lot of fish on their list of fish which they can eat, even their own kind.
Koi do best in a pond at least 12ft long by 6ft wide and 4ft deep or larger. Non-fancy Goldfish do best in a pond at least 5ft long by 3ft wide and 3ft deep. If your area never freeze's during winter time then you might be able to opt for a shallower pond. Both koi and goldfish do best in filtered ponds.