I kept a few yoyos for a while back when I had a pond snail population explosion. I couldn't tell that they made a dent in the population much but I didn't notice any fin nipping. I have found that assassin snails work extremely well for controlling the snail population but I can't seem to keep many alive for very long with the levels of CO2 I run now
It takes quite a few assassins though to effectively control the pest population. I used 12 in my 30 gallon.