Light Vulnerability (Ex): When exposed to sunlight or to light equivalent to a daylight spell, a shadow hound is slowed (as the spell). This condition persists until the shadow hound leaves the area or the light is extinguished.
CR 6 sounds ok, but maybe a tough CR 5. I think the mastiff's SAs are maybe a little better.
Often NE is good.
Advancement: 7-9 HD (Medium), 10-18 HD (Large)?
200 lb?
Common, I guess.
Usual magical beast training stuff (ie, higher DC)?