I asked one of my friends and this is what she said. She used to have tons of turtles and animals. Hope this helps...
The turtles will hibernate in the winter, but as they grow larger, I would NOT leave them with the koi. They will eventually rip out the stomachs of the koi (eating them) and it will be a heart-breaking experience for them (the koi AND the owners of the koi).
Now, whether or not she keeps the turtles is a personal choice, but I would NOT let them stay with the fish at any time, unless they intend to let the turtles eat the fish - because THEY WILL!
It is hard to breed turtles in captivity, but it can happen. Usually, they will eat the eggs - but let just one little egg go unnoticed, and you will get a baby turtle if it incubates. The turtles could also eat the baby turtle (again with the trauma thingie), but nature tends to take its course anyway. It either makes it or it doesn't. Sounds cold-blooded, but so is nature - and so are turtles.
Christie, I wouldn't take on any more than you already have, or you will be inundated like we were. They are cute, they are fun, but get too many and they get territorial and they STINK to high heaven, not to mention eating a LOT. Learn to politely say "no" to offers of someone else's unwanted pet. You'll end up like me (not that Michael would let you!).
As far as the males mating, well, boys will be boys dontcha know? Accidents happen.
In my humble opinion, turtles are best left in the wild, where they can do what turtles do when turtles want to do it. They are neat for a while to watch, but are they really happy having you ogle them all the time? I think they would rather bask on a rock in the middle of a lake where there is no human intervention and catch fish (and, yes, baby duck feet) any time they want and not upset their owners!
ALSO ...
In order for her to safely hibernate the koi in wintertime, she needs to either:
Put heaters in the pool to keep the water above 40 degrees Fahrenheit OR
Make sure the pool it at LEAST 3-1/2 feet deep so the koi can get to the bottom and stay warmer. Ice should never be able to form around them, fencing them in!!
A 29-gallon tank is certainly not large enough to house koi. Our 125-gallon wasn't even large enough.