I've not tried them, but I've read that there are issues with the Magpul 12's and the 26.5, something to do with the ambi-slide stops, I believe.
I finally got around to trying the medium beavertail on my 26.5 since I like that set up on my 19.4 so much, but removed it again after only a few presentations. I'm sure it would be pleasant to shoot in that configuration, but I didn't like how it changed my master grip on the draw or in SHO dry fire, which composes about 80% of my dry work.
This thing has definitely made me want a 19.5, of course.