John,
I am by no means an expert, but I do have goldfish in my 1000 gal. pond.
From what I've read, goldfish need about half the room koi do - about 50
gallons per fish assuming each one will grow to 10" at most. Using that
formula 6000 gal. should accomodate 120 goldfish.
As for adding dechlorinators to the water - I added nothing to my tap
water when the pond was built. Just let it sit half a day and added my four
6" goldfish who are still alive 4 six months later! I have heard that
goldfish are more indestructible than koi though.
Hope this helps.
Craig