Never thought about keeping these as pets before.
Used to
live by a river, and these things are really easy to catch. You just stand down stream so you don't disrupt the water flow (which kinda worries them so they disappear) you usually find them hiding under rocks and stuff so flip a rock over carefully, then move your hands toward it slowly and then the final movement is a quick cupping of the hands around it. And even if they're able to grow as big as 7 or 8 inches, I never saw one bigger than 4. So I imagine that's more the average.
A friend used to use them as bait when he went fishing, because they were easy to get, and there were loads around, and they were free.