That is an isopod, related to sea lice and pill/sow bugs, sometimes called rolliepollies, they are harmless and eaten by many types of fish. They feed on plant debris and other organic detritus and depending on the species range from 1/8" to 3/4" in length. Not a problem to worry about.
As far as feeding snails to loaches, I wouldn't just because the snails will usually out breed the demand for them and then your tanks are over run with as well as your pond. My method for snail control is physical removal of snails and snail egg masses. Just spend a few minutes each day picking out adult snails and wiping up the egg masses that you find with a clean wash cloth. You won't get them all but it should help.
There is also a product made that I believe is called snail-rid that is made to kill snails, I have never used it, and don't know if it would be safe for your pond.