Kinda long story, but here goes:
I've had the B&W for 1 1/2 years, then she paired up (maybe) with a false perc for about 8-9 months. I got bored with my tank cuz both clowns ignored the BTA I put in, for a month. I gave the false perc to my LFS.
The GSM has been in there for about two weeks. (hosted immediately, the B&W did the next day) The B&W is bigger. When I first introduced the GSM, the B&W snapped at him a few times, even managed to split a lower fin, but that was healed in a couple days.
The GSM is very friendly, (also has his own RBTA) he's always going over to the B&W, but she is kinda un-friendly, and usually pushes him away. but no damage on the GSM since.
It's pretty tough to predict how any clowns will react to another one. I know maroons can be very territorial/aggressive. Also, you have a little less room than mine do, so it's a tough call
If it were me, I'd talk to my LFS about it, and make sure if yours attacks the new guy, you can pull him out and take him back.
hope that helps