Im sure you already knew this, but the footprint is more important than gallonage, but 48x24x24 (glasscages.com) could probably hold your 7 tightly. Larger is better, especially with that many axies. As far as I know, excessive breeding could soon become a stress issue to your females if they live with the males for a long period of time, especially with many males. I would buy whatever size tank you can get that would be big enough, then put an acryllic divider down the center for males and females. Maybe someone else will chime in, but that is my opinion. Hope this helped.
Also, it wouldnt hurt to quarentine the new 5, just incase.
Mark