I believe that the Burris website has a list of all of the adapters and which guns they fit. Maybe the guy that you talked to didn't know. But I would think that if he works for them, he would or should, know one way or the other.
That said, DO NOT buy the Fast Fire II site.
At one point, I purchased one each of the FFII and FFIII. One with the Picatinny mount and the other with the Glock mount. The FFIII allows access to the battery without removing the site, but it uses a less common battery, CR1632. You can get them, but I happen to have a ton of the 2032's that are more common in a lot of dots/sights.
But, the most important thing is this. The FFII battery contact is connected to the circuit board by one very small solder joint, about 1/16" in diameter. When I put in my battery, when I first got it, the spring-like contact broke right off at the very small contact point.
The FFIII has several (3, maybe) contacts around the perimeter of the battery compartment. All of them have a much better/secure contact mounting system.
I sent my FFII back with the $30 difference in price and got a second FFIII to replace it. Both have been very good.
Well worth the few extra $$ for the FFIII. I don't know why they still make the FFII, I think other brands use them and put their own labels on them.
Good luck.