You have some really nice Comets and Shubies in there, Keith. I'm not going to comment on the turtle. I can only tell you I sure enjoy my little guy. He figured out really fast about coming out when I go out to feed in the mornings. He's back in the goldfish pond again, or at least I'm pretty sure the one in the koi pond last night is the same one in the goldfish pond this morning. He's really neat. I toss some food on top of the lily pads, he hears it, comes looking, and climbs up onto the pads to get the food. But, the 3 koi that are in the goldfish pond also hear it, and I swear they can smell the food on top of the pads! The one koi bumps the pads until the food rolls off into the water, or he climbs onto the pad, sinking the edge, to get the food. Silly fish, cause I feed them first so they should be full. I guess he has a bigger appetite, or it's a challenge to him.
The only reason I would say no on the turtle, is that mine has obviously gone from pond to pond already, and that means he's climbing out of one pond, over the wall and into the other pond. The stream would be a climb to get into, and still a wall to get into the koi pond, since they are really not attached as far as water flow. So, even though he is only about 4" long, he is a good climber. Other than that, I would say go for it, especially if you can find one pretty cheap.