I think you're now going to have a hard time totally eradicating the snails. You can keep picking them out and/or try putting in lettuce leaves at night – in the morning, supposedly, they’ll be covered with snails and you can pull that whole thing out. Some people don’t mind snails. But I always quarantine new plants for that and other reasons. Then if I see snails, I can treat with alum. You can’t really treat a 55 with alum – well, you could except it would take an awfully lot and then you’d have to basically tear it down rinsing the stuff out. Perhaps (just speculating) if you took the plants out and treated them and gravel vacuumed well, since there wouldn't be plants around for them to hang out on, you could see them better to pick out and eliminate.
As for your other little bugs, I don’t know what they are, but they’re not parasites, just nuisances. As with most such things (nematodes, planaria, etc.) they are usually seen in abundance only if you overfeed and underclean... however yours is a new set up, I don't know why'd they'd be covering the leaves except they just hatched... when their food source dries up, so to speak, I think they will be unnoticeable. Not gone, just unnoticeable. Do they gravel vacuum up?
For future reference, as justintoxicated indicated, plants can be treated for alum (aluminum sulfate) to get rid of snails and eggs. You can buy it at a grocery store (spice section, most commonly used for pickling), it's a white powder. These are the instructions I have copied down from somewhere a long time ago: Use 1 tablespoon of alum per gallon of water. Soak the plants a minimum of 20 minutes, up to a maximum of 2 hours. A two-day soak will kill snails and their eggs. Avoid using this method on extremely fragile plants. Note, after I rinse really really really well in multitude water changes.
