I have heard over and over that if you had a setup where you needed to add another rat, just give up and and get all new rats because they will kill each other.

I'm not sure if this meant if you had a colony of girls you can't add a male if another dies you can't replace it with another male BUT if the belief WAS that you couldn't add another male to a pre existing female colony then I believe I may have proved that belief false.

I have 30 tubs going with my ASF's. 2 females to 1 male. I set all my females up in pairs a few weeks ago and just last night, I added a male to each of their tubs. I had 29 tubs where the males and females were 100% fine. Some squabbling, some mating, and some arguing, but nothing serious. I had one tub where the male was killed asap. I believe that tub has a female that is already pregnant and this is why it happened.

But 1 death out of 29 tubs sounds like a success to me. So if you're wondering how it will work putting a new male with a pre existing colony of girls, give it a try. For me, 29 out of 30 times it will work!