I have a ~2-3mm fine grained natural gravel in my 55gal w/ dojos. Mine prefer to lounge and weave through the hardscape, anubia and java fern, but they can easily dig through this substrate as well.
To the OP, you said you live in NY (I do too) and would be keeping them in a pond. What about winter time? It's said that weather loaches are more temperate than sub-tropical, but I'm not sure they'd survive in say ~35Fdeg water.