well, the worst that could possibly happen is a collapse of the tank's ecosystem. I would think that as long as you make sure the plants and snails are clean, they won't cause a catastrophic ecosystem collapse.
EDIT: another thing to keep in mind... I don't know about up around Crown Point, but here around Indy there are still occationally small amounts of toxic chemicals found in the wildlife (specifically in the White River) due to the industrial spills (i believe some mercury and something else) and subsequent fish kills that happened in the late '90's. Typically it's not significant, but it only takes one still-contaminated plant to ruin an entire tank.