I have a 3 year old RottX female, a 12 year old ACD (Australian Cattle Dog) male, and a newly adopted, 2+ year old Rottweiler female.

I know about the problems that two Rott females together can be, and after a rocky beginning <img src="/ubbthreads/images/graemlins/shocked.gif" alt="" /> I've had good results with getting them to behave. They have even begun to play, the RottX tries to get the Rott to run and play, nudging her neck and sprinting around. She will sometime engage and chase her about, and they seem to be getting along, although if they seem to get too excited I call them off and make them stop. The ACD tries to get into the fray and causes some problems, he likes to double-team the Rott, air-snapping and barking at her, so I call him off and don't allow him to play at all. He did this against the RottX when my little Blue Heeler was still alive, but now he backs up the RottX. <img src="/ubbthreads/images/graemlins/crazy.gif" alt="" />

Should I allow the two girls to play? They had a couple of nasty interactions when they first met, but with some training, they have been getting along well enough, even to recently travel together on a 750 mile road trip, without incident!

Or, am I just asking for trouble by allowing this? It is good for the Rott, who is about 10lbs overweight. And it is great fun to watch, they're such goof-balls.

Any advice?

I would allow them to play supervised at first. Which dog has the alpha position? If it is one of the Rotts and the other one respects that than I doubt you would have a problem. On the other hand, I would work on teaching the cattle dog the "off" command. Once she has that down I would see how she is once the playing begins. Use a really long lead so you can pull her off if needed. Just a few thoughts. Those cattle dogs are bread to herd and running alongside nipping is in their blood, so not sure if my suggestion will work but worth a try.