I find it helps when stocking a smaller tank to think of the fish as inhabiting different "levels" and to only have one type of fish for each level. Thus, your pandas are your bottom-dwellers and its best to not have any other type of bottom-dweller. Your harlequins will stick around the middle and when the tank settles down you may be able to get something else that favours the upper level of the tank. For example I have a 60L tank with GBAs lurking in the depths, pearl danios in the middle and gold gouramis that like the top. I suspect the harlequins might be calmer in a school of 6 or more but I'd wait until any water quality/health issues are under control.