IMO, the three brands mix rather well excepting perhaps Pendraken as they are almost a head taller than the other two.
As usual click on the pics for a bigger view.
They are from left to right:
Orcs: Copplestone's, GW and Pendraken.
Humans: Copplestone's, GW and two of Pendraken though the human belongs to a historic range which are smaller than the fantasy one.
Dwarfs: Only Copplestone's and GW, two of each.