I have four axy's under 15cm in a large tank filled with driftwood rocks and plants. This gives them loots of places to hide. During the day it is normal to only see 1 or 2 out and about, but come feeding time (8pm) they all come out and sit along the front of the tank.
So its normal for your axy to hide. If one of them does lose a leg or gill don't worry too much as it will grow back fairly soon, if however one is the subject of repeated attacks you may have to separate them.
Hope this helps, .