I've had brown kuhlies for years now and love the guys. They're pretty hardy if their needs are met. My oldest are something like 5 and still going strong. They do best on a sand substrate as they like to dig in it for food particles. They can injure themselves on gravel. I've noticed that mine that lived on gravel for a while have shorter barbels than the ones that have only lived on sand.
They do need a lot of structure on the bottom such as rocks, driftwood, and plants. Mine haven't been burrowers but they do like to get under things that are close to the ground like low-hanging leaves and rocks with little outcroppings. They will be much more visible if they know they have a lot of hiding spaces.
They are highly social like all loaches and do best in groups of 6 or more. Really as many as you can fit in the tank as possible is best. In a 20 long you'd want about 8 of the brown kuhlies or 7 of the striped kuhlies as the striped ones get about an inch longer.
Very few are captive-bred. Most of them are wild-caught, so they will be more sensitive to water parameters than fish like mollies and bettas. As far as some species being harder to find than others; it all depends on your local suppliers. None of mine got in any Pangio species, so I ordered online. All the fish were safe and healthy.