Put as many stones in as you can...I cannot say how many exactly. Would be a good idea to have them under strong light, perhaps they will develop some algae.
As for the "fin ventilating" -- perfectly normal. Hillstream sucker loaches do this, and they also do it when sitting on stones.
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PS. Hillstreams are pretty sensitive to infections, so regardless of where you get the stones (store, beach, another tank), disinfect them before putting them into the tank. Boiling usually is the easiest with stones.