Koi can adapt to an aquarium environment but it's not recommended for a few reasons. First one off the top of my head, water volume, 55 gallons isn't enough to start young koi off on the right track.
If you choose to raise very small koi in a 55gal you should be aware of the pitfalls. It's work. Water change outs are a necessity on atleast a bi-weekly basis. Limited feeding to slow the water pollution rate(which means slower growth rate). No gravel substrate is highly recommended. Ample filtration is a MUST.
Young koi can and will grow very very quickly when given the opertunity, meaning an excellent environment in which to flurish.
A 300 gallon stock tank would be more suitable imho for 3-5 very small koi (under 4 inches) for the short term. 150 gallons minimum for a couple. A stock tank is a good investment, good for young small koi short term and also good for adult koi to be housed in temporarily if there comes a time for needing treatment for whatever health issues they might face.
4 minimum to 8 ft depth should be considered for pond depth. Square footage at the bottom of the pond is as important as surface area, especially for overwintering. There are many commercial Deicer's on the market(price varies), from 1500w to 100w, to keep a hole open in the ice. There are a few DIY Deicers as well. Something else to consider along with deicers is the use of a very small water pump or air stone and air pump, there needs to be very slight circulation happing over winter to help with gas exchange. Many ponders also opt to cover their ponds with a greenhouse type structure to keep the winter elements off, wind and snow can have varying degrees of effects on unsheltered ponds most of which are not good.
Nuff of my babble.... good luck!