I currently have a 12.5" yankee hill slr handgaurd i may use on a pistol build. The handgaurd is 12.5" at the top and with the slant (slr) is 13.5 at the bottom.
My ? is could i get away with a 12.5" barrel with a kak flash can and have it long enough to keep expelled pressures off the handgaurd?

Should I just get the 10-11.5" barrel and get a new handgaurd?

I dont want much muzzle/barrel showing past the handgaurd.

No direct experience here, but on my 16" barrels with 12" (rifle length) rails there is approximately 3" of exposed barrel before the muzzle device begins.

Is your 12.5" rail a rifle length?  Not sure if that is rifle or slightly extended.  

My 11.5" barrel has a midlength rail on it, and I like it a lot.  10" would probably look better though.  I wouldn't want a 12" on it, but I just run a flash hider.

I don't recommend having your fingers that close to the muzzle, so if your handguard/free float tube is close to the same length as the barrel, you definitely want to either use a 4-5" shorter handguard or a large hand stop of some sort. Fingers don't grow back.