That none of their disciplines have much NEED to channel, whereas the Battle Ajah and Gentling Ajah definitely do, and the Blues causes tend toward similar challenges and confrontations. But Grays are negotiators rather than competitors, so as long as Compulsion remained forbidden channeling would always be incidental to their role; it might help if attacked in their travels, but a Warder would probably help more.

The best candidates are Whites and Browns, because their disciplines are largely devoted to contemplation rather than action; the less action, the less need to channel, and the more concentration on other things, the less desire to channel. Perhaps if researching things closely tied to channeling, but otherwise people who spend their lives studying and thinking locked away somewhere isolated precisely to prevent distracting interruptions have few needs to channel, let alone channel large amounts of the One Power at any given time.

Adeleas is the best choice, IMHO. She fits the bill of being "old school" (unless that means AoL, which is unlikely) and her Green twin could well have been forced in her younger years before she became practically a Brown as well. While they moved around together in their research, they were also often secluded when pausing in their travels, and if any defensive channeling were needed one would expect the twin from the Battle Ajah to be better trained for it than the Brown, leaving Adeleas little cause for such acts unless Vandene were overmatched.