Koi can hibernate if you prepare accordingly.
About the indoor keeping: First off, 50 gal won't suffice for an adult. A RES for example is gonna need 100+ gal, smaller species still around 70. If you're keeping it heated indoors it'll probably take quite some time until your pond is warm enough for it to be reintroduced, which will probably lead to it being kept more indoors than outdoors, dunno how much that would bother you. You can also hibernate them in a fridge. I don't know how harsh the winters are there, but there's probably also species that could hibernate well outdoors, fridge is definitely safer though.
Turtles are perfectly fine alone. Keeping male + female for instance will just lead to the male constantly bothering the female, trying to mate.
Turtles can definitely be assholes. I wouldn't be able to keep my yellow slider female with any fish. I'd suggest smaller species like a musk turtle or a male map turtle.