The voicing between the NrT and Ribbon version is nearly identical. This was actually a very high priority for us. What I had originally recommended was to try and avoid going with the standard NrT version of the tower center and ribbon left/right. This is not due to a timbre mismatch. It is more so that the ribbons are able to reproduce vocals with a smoother presentation than the dome version and vocal reproduction is critical to the center. I would not want the ribbon mains to outshine the center when it comes to vocals.
Of course, it all depends on priorities. If music is higher priority than home theater, it is absolutely OK to mix and match