1. All levels, looking at my tank right now, a group are hanging out at the top, some are swimming in the fake Roman columns (middle), and one is nibbling in the gravel. They are very active and pretty much everywhere.
2-3. These two questions are very related since 10 gal is not a lot of room to add much else. Tiger Barbs are a shoaling fish... that is, in order to be happy and healthy, they really need a significant number of theor own species. If they do not have some friends, they normally act nippy and aggresive toward other tankmates. It is generally quoted to keep at least 6 tiger barbs. But, they also get to 2.5 to 3 inches big. So, about 6 is all you can fit on one ten gallon. No other fish at all.
Now, if you can afford it, since they are such active swimmers, they would really appecriate a longer tank to zoom around in. And, if you get a larger tank, a community of 6-10 tiger barbs will typically keep to themselves, and other fish may be kept with them.
But, in summary, if only have 10 gals, I would recommend six tiger barbs and six only.
[edit] snuck in there digital_run just as I was writing B)
