[toastybiz]

Gnats. If these are indeed fungus gnats, you've got the right idea. Fungus gnats potentially need to be dealt with on three different levels -- eggs, which can be suffocated with a layer of sand (good call on that one), larvae, for which put some potato slices on the soil, the larvae are drawn to it, then in 4-5 days throw away the potato slices, and flying adults, for which neem oil, tobacco juice, or just sticky fly traps will work.

Worm. If indeed a garden-variety earthworm, yes this is good for your soil. Earthworms aerate the soil and keep it from compacting. Don't expect much benefit from worm castings, though -- it would take a bucket of worms to produce enough castings to make a difference.