From the "more information than you really wanted" department:
Techno-mages must be vertebrates with a generally humanoid form and nervous system. The tech that gives them their powers must conform to this basic standard. The race must not be inherently telepathic, because mages and telepathy don't get along. Hmm, I wonder why that could be...?
Thus, for example, the Gaim can't be telepaths because their insectoid forms don't have the right kind of nervous system, and the Vree can't join with the tech because their telepathic nature makes them incompatible. Historically, most of the other B5 races have representatives among the mages, and there are rules in the book for racial benefits related to magery.
By the way, the book also has rules for two new races, the Kaitay and the Taratimude, the latter of which has Techno-mage as a favoured class.