I'm with Samophlange, in that while the old WEG Star Wars stuff was good, it shouldn't necessarily be held as inviolate simply out of sheer nostalgia.
As much as I enjoy Zahn's work in the post-RotJ era, I'm willing to accept that the new movies may very well invalidate some, if not all of what he's written in regards to that era. He'd already had some of the stuff in Thrawn Trilogy invalidated by the prequels, as have other authors; most folks figured it'd been more than 20 years between when the Clone Wars ended and the Original Trilogy began, or that Luke & Leia's mother had survived for at least a little while after the twins were born.
As was noted in a similar thread over in the AoR section, The Force Unleashed (aka the last big multi-media event that Lucasfilm tried to organize) has, per Pablo Hidalgo, already been moved to a non-canon status in regards to how the Rebel Alliance came to be.
There's also the simple matter that what WEG created is, in RPG terms, ancient history. It'd really cut down on the creativity of new material like Star Wars: Rebels if they had to remain slavishly devoted to huge swaths of stuff that a bunch of third-party writers created well over a decade ago.
Yeah, Zeb's been tagged as a Lasat, which was a label that WEG applied to an old concept drawing, something they did a lot of during their ownership of the license since it was probably cheaper just to slap a made-up name onto an early concept drawing (Z-95 Headhunters came about by way of early X-Wing concept drawings as another instance). But that doesn't mean the Lasat species in SW: Rebels has to follow the same template that WEG laid out. I glanced at the Wookieepedia article, and the Lasat as described there were indicated to prize cunning and sneakiness, where Zeb seems to be of a warrior-race type of background that's very direct in dealing with an enemy (he's pretty much a Hired Gun/Marauder in my eyes).