I have a friend with some red ramshorn snails and she states they eat them. She also has some larger apple snails that do not seem to be bothered by them.
I have 3 kuhli loaches with 5 panda cory's and they do not cause any problems with them. The only draw back that I have seen with these loaches is that they are very rarely seen. They seem to hide in the gravel or under the driftwood most of the time. The only time I notice them comming out is at feeding time.
kuhli loaches do better in large groups that may not be possible if corydoras are already in the tank (if the group is sufficently sized and the tank is smaller)
Kuhlis are very peaceful and will get along with the corydoras. Please do note to buy 5 as the minimum recommended. Lesser than that does not encourage confidences hence you see them less frequently than a very large group. If your tank permits space, do buy no less than 5.
And in my own experience,....a Kuhli with no confidence, is a kuhli that ya might as well tape a picture of him to the side of the tank,...b/c you RARELY if ever see him.
I've got a ramshorn snail with my kuhlis and they don't pay any attention to him. As a matter of fact, I got the snail as a freebie when they dipped the kuhli's from the petstore tank.
I'm actually skeptical with the claims of kuhlis eating snails. I've never seen them eat snails except the Botiine genus. My theory is that they are only consuming the snail eggs thus preventing more snails from hatching. If the snails are MTS, I doubt there is a chance these ones will be eliminated.