Well i'm certainly no expert, i have only been keeping pygmies for just over a year or so now. But in that short time i have kept over a dozen at least, of three or four different species. I have i feel, learn't quite a lot in that short time though. Although lots of people seem to have success keeping pairs, trios etc. My results trying this, have resulted in losses. Not just of males kept together, or male & female, or even male & two females, as is often suggested. But also two females together has resulted in the loss of one. I have tried this in decent sized, heavily planted tanks (exo-terra's) too & still lost animals. Have not lost an animal housed alone to date (touch wood) yet. Have spoken with other much more experienced keepers, who have had the same. But at the same time, others have not had any problems. Just can't seem to find exactly why some can keep groups & others can't. Does seem to be lots of people lose them for no apparent reason though. The fact that most are WC & have a short lifespan, could be one reason. It's almost impossible to know the age of WC animals & at best is a guess, usually based on size. I just see mine do better when kept alone & if attempting breeding. Just introduce as & when.