Hello Ella, goldfish can go in with koi, BUT it has to be a koi pond, you can't put koi in a goldfish pond. Which do you want to do? If its just for one fish and a friend has koi ask them will they take the goldfish, and when are the visiting times.
For suitability it depends on is the area under or near trees. To construct a pond properly is a lot of effort, and can take some time (and money) more information is at the start of this topic click me Yes, a pond should ideally be cycled but the truth is no one does.