I wouldn't do koi, unless your pond is 1,000+ gallons- they really do get HUGE (3, sometimes 4 ft long), and probably shouldn't mix with orandas due to potential aggression and territorial disputes, and the orandas not being able to even get away from them, much less defend themselves. I'm sure there are people who *have* kept orandas and koi together, but, to my understanding, it really isn't a great idea to try it. More comets would be good- if you don't get winters dropping below 40 or 50 degrees, you could do fantailed goldfish in there- they don't get as big as the single tails, and would mix very nicely with orandas. Koi can be done in smaller ponds than 1,000 gallons, I am sure... but, I wouldn't get too many of them, and I wouldn't put anything slow moving like double tailed goldfish or orandas in with them. Comets should be OK with them- the fantailed varieties get bullied way too easily to handle living with koi in closer quarters, IMO.