nice tank, you can keep multiple firefish in your tank. With purples you should only have one I believe. I had 5 firefish in my tank, one made the leap out and perished, another perished in the tank, a third made it into the overflow and down into my sump where he now lives (can't catch him), and I still have two in the display. I believe it is best to keep them in odd numbers also.
Hope this helps.