You absolutly have to move that koi they do get 2-3 feet long! They need to be in ponds or HUGE coldwater aquariums, find him a new home or build him a pond. The goldfish would be fine by himself in that 20gal but no one else can fit. As for the other tropicals I don't know if they will get along with the fish in your 250 but if they will that is a good place for them. If you can find new homes for your koi and gf then you could just add a heater to your 20 and add some more tropical fish. Like your big tank! 
