Look like any of
These? If so, they're corydoras catfish, often called "cories". They're generally about an inch and a half long. Probably what you got.
Or more like
one of these? These are called generally called "plecos" (or, by the superstitious, "pl*cos". They're noticeably bigger than cories.
Can't tell you much about plecos, but if they're cories, all you need are some small spaces. I've generally made "caves" using the following methods:
1) Get two pieces of flat rock (like slate), and lean them against each other to make a tent.
2) Use some larger (say, egg-sized) rocks to make small "caves". Put two down, a little apart from each other, a third on top of the first two, and maybe one covering the back if you want. Just ake sure the rocks are pretty stable.
I've heard of many people using half a clay pot as a cave, too.
Or, taking a short piece of PVC pipe, and using aquarium silicon to cover the outside of it with gravel.
Also, if you have plants, sometimes that's enough, too. Seems to me that they just like to get out of the light occasionally.
Hope that gives you some ideas.