Creating dimorphic clans is a lot of extra development, especially if you consider the lion-like race (assuming the leak is accurate) with male and female counterparts for each, that's basically the equivalent work of designing three races. There's a reason that the Au Ra races are (mostly) just palette swaps of each other, that's because it creates less design work. As is the only dimorphic race we have right now are the hyur, and when you consider the highlanders were male only to start with, that makes more sense. Going into 2.0 they didn't design a dimorphic race, they just added a female option to the highlanders and classified them both as hyur.

Creating a tall and a short version of viera isn't impossible, but you do have to consider in how much additional work that would have been for the development team in determining how likely that option is when it's implemented. Based off the trailer we've seen, I'm willing to bet we just have one clan that's darker skinned and one clan that's lighter skinned rather than a tall clan and a short one. But I could be proven wrong once we get all the expansion information.