yes it does tickle quite a bit, it still freaks me out a little every time but you get used to at is you do it more, and yes shrimp tickle the same but they prefer the legs over the feet. You find them more on ocean side beaches, where I came from before I moved, you went in shallow water walking on sand dunes where the water is warm 5 minutes and you can see them eating on your legs but they are tiny, like half a centimeter or smaller.

I never experienced nail problems either but like I said, I do it in the wild not a salon, maybe it happens if they feed from someone's feet that had contamination than they can transfer? I do it all summer long I just sit at the shore with my feet in the water and next thing you know there is a whole bunch and the minute you move your feet they all go away, we have really clean rivers tho with lots of fish.