Pond snails would be a small sourceof ammonia, but probably not significant. I also think that they would not survive the spikes of a bacteria cycle.
If you, for whatever reason, can't use a substrate and plants, you could put in some plants in plant pots and if you use stem plants you might be able to do a "silent cycle" (with plants).
Otherwise, I would do an added ammonia cycle or even better, seeding from another tank that's already completed cycling. Probably the fastest way to non-plant cycle.
Btw, how many days did it take to complete the plant cycle. If you have adequate lighting to grow the plants, it shouldn't have taken long, and I think that's the way it worked out. Meanwhile, the bacteria are slowly settling into the tank slowly.
MY TANK: Planted 10g; 2 x 10W CFL; Fluval U2 internal filter; MGOCPM/black sand cap
A Silent Cycled Tank