I aquirred a hatchling painted turtle in April and also wanted bottom feeders to help clean up her mess. I purchased two emerald green cories and two ottos. As of today both original cories are alive and very well! I have one original otto from my first pair as the turtle got one of them when she got large enough. But i bought another and its been fine for two months. My turtle's basking spot is such that the fish can hide under it without the turtle being able to get at them. So if you have a similar set up in mind it is possible. My fish are now wise to the turtle (they know there is a predator in there and take appropriate cover when she is swimming around. I dont know how long your tank has been established but keep in mind how messy turtles are and if your tank is not properly cycled be prepared for LOTS of water changes as ammonia builds up fast with turtles, let alone more fish. And also, my tank is cycled but the nitrates!!! i can hardly keep them under 40ppm, even with threex weekly wc and gravel vacs. Turtles are lots of fun, but lots of work!
just thought id add my two cents.