I wouldn't call that large, and that's about the minimum for which the Society for Human Resources Management considers that you need even one full time HR person, but never mind, it's large enough for this purpose.
No one promised you a workplace free of stress, free of any kind of negative atmostphere, or free of people being mad at you. No one every promised you a rose garden, as the saying goes.
"ZTR has a disabilty - therefore I'm going to give him a hard time" - illegal.
"I liked the job I had and because of ZTR, I have to do a different one - therefore I'm going to give him a hard time" - legal.
Remember, your employer has already gone WELL beyond what the ADA requires of them. They created a new position for you and they bumped another employee out of his job in order to do it. The ADA does not require that they do either of those things.
Just saw your updated question. FMLA does not give you the right to just walk off the job. It's unclear whether you notified the GM that you were leaving for reasons attributable for FMLA or whether you just said, "I'm leaving" and left. If the first, then that's legally questionable, though I very much doubt that a single instance under the circumstances you describe will be actionable. If the second, then yes, they were within their rights because they did not have reason to think that it was protected time.

