Before my 55gal became "Bubba's" home I had one kuhli loach in there with some angels. I knew they did better in bigger numbers (from what I have read onling and been told by other aquarists) but they are amazingly hard to find around my area. So whenever I would see one at the LFS I would buy it and add it. Eventually the more I added the more I saw them come out and stay out and play.
Seems like the more hiding places I added the more they didn't use them. In my theory it was because they knew where they could retreat if need be and they didn't have to and felt more confident being around each other to venture out a "play" more.
When I only had the one or two in the beginning I rarely saw them. They were always in the sand or caves. When the number finally reached around 5, I saw at least 3 or 4 out all the time at the same time. There are now in my 280gal having a blast!