All regular Army regiments were undermanned. Details from volunteer regiments were used to fill artillery batteries, but I have never read that they were used to augment cavalry regiments. What happened in the Army of the Potomac was that the Reserve Brigade of the Cavalry Corps, to which the 4 Regular Army cavalry regiments in the East were usually assigned was so reduced that whole volunteer regiments were assigned to the brigade. The 6th Pa was with the Reserve Brigade from beginning to end, the 19 NY for a few months and the 2nd Mass for the last year or so. At the end even with the 6th Pa and 2nd Mass the brigade mounted less than a thousand men.