I don't recall seeing anywhere that he references what brass he uses. His instruction page is found here.
However, just going by what I know about the process, I would imagine that using mixed brass would not be the best idea. If the volume is close to the same then you shouldn't have any trouble but if the case volumes vary by much then you'll be developing different pressures, possibly effecting how well the powder burns and definitely effecting the muzzle velocity.
Whether or not it would be enough to matter, I have no idea. Probably, in a bench rest quality gun with a bench rest quality shooter, it would matter. For me, I would guess I couldn't tell. For you...?
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.