Well, Amit, I guess you must have a planted tank, like me. If given a planted tank, CPDs do like to hide. I have some smaller tetras that seem to join in with the CPDs also. My cardinal tetras often hide with the CPDs, but overall come out more often. I have all small, passive, peaceful fish, and, yes, they often hang out with each other.
By the way, I think your CPDs are just doing what comes natural to them. My CPDs are healthy and happy (breeding a lot, though my other fish eat the eggs and fry as fast as they are produced), but I do get tired of seeing so little of them. I’ve decided to remove my CPDs from my 50G display tank and put them in their own 20G tank (I have 18 of them, plus a 1/2″ fry growing in another tank). I always have plants in my tanks, but I am going to try to plant the 20G in a way that the CPDs cannot get completely out of my sight. We’ll see how it goes.