Bridge mounts are exactly what they seem.
Pros - easy and cheap way of getting a red dot on any railed gun
Cons - sight sits pretty high above bore axis, some people thing it looks cheesy, adds extra weight, almost impossible to carry.
As for OP, you can get any 226 slide milled to accept any red dot (pretty much).
You can also get a dovetail mount (don't quote me on that), cheaper and not as permanent.
The problem I see is that it is almost impossible to buy another legion slide if you mess this one up. Unless someone else out there is willing to sell you theirs. If you know 100% this is what you want to do with your legion, than go for it. Otherwise if you're not sure, I would personally buy another 226 slide, maybe even pre-milled, and just get it cerakoted sig dark gray. Save you a lot of money and headache if you change your mind down the road. Plus no one will notice it's not a legion slide since the red dot will sit on where the legion symbol is.