Looking for blast forward and flash suppression on a 12.5" barrel.  Do the 2 piece cans on eBay work? Is there any other, shorter devices I could use?

Check out the Dpms Levang. I have both and prefer the Levang because it smaller/lighter.

I bought a KAK micro can for $20 a couple months ago.  It's a 1 piece and aluminum, but works pretty well.

It's the same weight and length of an A2 birdcage which is nice.

I feel as if a flash hider on a 11.5/12.5" barrel isn't bad at all. I tried out an 11.5" with a PWS Triad and I was pleasantly surprised. The Smith Vortex probably works even better.

Also adding a full size Krink/Flash Can to a 12.5" barrel puts you at about 14.5"-15" of over all barrel length. Id rather have a pinned 14.5" rifle at that point. Unless you want to keep it a pistol for ccw reasons, I wouldn't bother with a longer muzzle device.

If you do decide on a linear compensator I would go with the Kaw Valley, BRT Linear Comp, or a KAK Can Micro.

I put one of these little guys on my 11.5"
$30 or so, off Ebay. Seems to work good outdoors. Still loud as heck inside.