For what it's worth, my 21se came from the factory last year with a fusion Ms ra-55. I ended up replacing it with the Ms ra-60, which is an exact fit for the existing cutout/holes. I swapped because the volume control was acting up...probably could have gone the warranty route but I wanted to add the wireless remote (ant+) which the 60 allows (the 55 did not).
Note that install did require me to make my own harness but that was pretty simple (the 55 had a hardwired pigtail/plug that I ended up cutting off and using to splice to the one for the 60. Colors were all identical, so it was simple enough).
As others have noted, if you want fusion to play with your Garmin MFDs you need to get one with NMEA support...wasn't worth the added expense in my case. I think the 70 and up gets you into NMEA support.