I had a purple, yellow, and desjardini newly moved into the tank for about a month before I put in my gem. The three original tangs would occasionally spar, but nothing serious. I put the gem in at night--the next morning there was some posturing when the lights came on, and the purple had it corralled into one corner of the tank. However, as the day went on, the gem got more and more bold, and would start giving as good as it got from the purple. All 4 are about the same size, ~3".
It's been a few days now, none the worse for wear on any of them.
Been thinking about adding an Achilles and PBT to the mix. Exciting times ahead!