Since it's 10 gallon per inch of turtle ( I think) and the biggest they can get is like 12" and one is like 5" and the other is like 7". They are in a 170 gallon tank.
It's good that you have a BIG tank.
Just like with fish, a simple rule like that cannot work though. Turtles grow 3-dimensional. A 5" long turtle is probably only 3-4" wide and 2-3" tall. A 12" long turtle is probably 8-10" wide and 4-6" tall.
I've never kept turtles.. except for when I was a kid with a RES in a little turtle bowl.... but if it's true that 10G per inch for a 12" turtle is the right amount, then a better formula using LxWxH would work better.
Using the estimated numbers I just listed above, the body mass for a 12" turtle would be 12x10x6=720 where a 5" turtle would only be 5x4x3=60 so you can see the HUGE difference between a 12" turtle and a 5" turtle. It's not just a little over twice as big... it's more than 10 times as big.
Same with Goldfish or any other large fish, it is best to use bodymass as the guideline... not just length.
As far as using the kribs as feeders.. like others have said, as long as they are healthy, they should not affect the health of the turtles. It would probably be better to breed them separately so you do not lose your breeding pair as a permanent source of fry.