I've been told that it is safe for frys that are one day old.
If the manufacturers told the whole truth about their products they wouldn't be able to sell them now would they?
Copper should only be used in a hospital tank, that will not be used to house animals for any other purpose. Copper is deadly to to inverts as mentioned, and is also deadly to fish with continued exposure. Also as mentioned, continued exposure is difficult or impossible to avoid if you put it in a tank you plan to keep up and running.
As far as snails, If you want to limit the population in instead of removing it. In addition to the already mentioned need to eliminate excess food, leave the largest snails in the tank and remove and kill all the little ones you can. The big snails eat more, and will reproduce less when available food becomes scarce.
As far as your loacch, he can be trained to eat snails if he doesn't currently. Smash the snails and let them fall to the bottom preferably near the loach (If he's scoiable at all) once he gets the taste of snails he'll figure out how to unwrap them himself eventually. But of course once he eats them all, then you will no longer have an indicator of overfeeding in your tank, so you'll want to step up your maintenance to be sure anyhow.
The only type of snail loaches won't irradicate completly is MTS's, but MTS's are seldom seen in the daylight anyhow unless population are really high, and your loach will keep populations in check at the least.