If the species all have a common ancestor, and are a product of Divergent evolution or genetic manipulation, then it should be conceivable for all of them to interbreed. Whether that offspring is genetically viable to have children themselves is another question. It might be why interbreeding is rare or frowned upon.

In regards to 'size', thats going to boil down to aesthetics and function. Where a horse has a large part, that stems from the physical limitations of horses needing reach, while human size proportions are likely related to selective breeding. If they have a common ancestor, it's possible for all males to have similar sizing, unless some of it was bred out. Not likely, since we see Lalafell women pining over taller men. And same in reverse for lalafell men.

And with real world example of woman with dwarfism having children, it shouldnt be a stretch for lalafell to have the same potential with any taller race. Though magic assistance might be a necessity in childbirth