12mm alli is pretty substantial, and I'd hazard a guess it would be fine, but I'm sure others will have a better idea than me!

Regarding the rails, I was actually reading about them the other day (can't remember if it was on HSM, CNCZone, or the Mach Forum!), and the general consensus is most hobby builds are majorly over engineered, as there was some big commercial machine running 15mm rails for the long axis. The only reason I was reading about them, is I've got plans for a new machine bubbling away in my mind, and linear rails are featuring.