I think that tradescantia zebrina and common syngonium (not the pink ones) are fast and easy to grow.
Certainly easier then begonias. Most of mine even from rhizome grow quite slow and are easy to kill.
Also smaller philodendrons like heart leaved and Brazil are easy to propagate and hard to kill.
I am not sure, but are you selling the plants that yr buyers might use in their tanks? If so, then whatever is recommended for terrariums and vivariums makes sense.
dendroboard forums mention all sorts of creeping plants like Ficus pumila, Callisia repens, Pileas , etc anything small that requires high humidity.
