It won't do any harm to the plants and will probably benefit them but plants roots bring oxygen down to the substrate anyway. The snails are great at stopping compaction and helping the sand structure stay more open, so in that way they benefit the plants.
Depending on which species of loach you have, the snails might not last that long. Bigger species like clown loaches or yo yo loaches can easily hunt them under the sand and devour them but smaller species are good at keeping numbers in check. In my brothers tank he has a group of botia striata and still plenty of mts but no large ones. He does probably overfeed the loaches to much though.