Culchie...i think i would stick to the carp family...koi & goldfish. altho a couple of chinese high finned sharks would be a nice addition. they eat algeas, and are hardy as well. goldfish dont grow as large as koi and are less destructive. they also dont make the wastes that koi do so you can keep many more in the same volume of water. goldfish come in a few varieties that can be as nice looking as koi. commons, comets. sarassas, and shubunkins. some comets, sarassas, and shubunkins can really resemble koi. sarassas look like kohaku with their white bodies and red markings.
ive been told and agree, that its not a good idea to introduce any indigenous fish to the pond. they may carry disease and or parasites that koi, goldies, or any other domesticated fish may not have immunities to. centuries of inbreeding, cross breeding, and mans general intervention may very well have compromised the immune systems of the fish as welll.