Idiots like this cannot win, you have rights, you had cleared it with his superior prior to the event, and the "transy" comment was pure gold for you as that is harassment pure and simple, especially if it was in front of other people (who may also act as witnesses).
Guys, I think we need to take another look at the shift manager's words (according to Scotcop):

This is not Jr. high, this is not show and tell, we are big boys here, and we are here to work, I dont care if it's Irish Day, I don't care if you are a transy, but when you come to work, you will wear this uniform, or you will be released.
If those words were presented in a lawsuit for harassment, I don't think the suit would be successful. It seems as though he chose his words very carefully to come across as an insult while not rising to the level of harassment or accusation. Notice he said, "I don't care if you are a transy.". By saying he doesn't care, he is saying that it's not the issue. Only the dress code is the issue.

To me, this shows that the shift manager put a lot of calculation into this. He had to have planned it ahead of time, since he apparently knew Scotcop was going to wear his kilt. He waited until the moment was right and then he attacked. And his words were carefully chosen to be insulting and degrading, while not being dangerous in any legal sense.

At any rate, I still think there's no cause and no standing for any type of lawsuit or legal action. But this guy is definitely a jerk who walks around with his little chest puffed up, thinking he's something special, and getting his jollies by tearing others down. People like that need to be knocked down a peg or two. As a manager myself (over an engineering department, not retail sales), I would never talk to my employees like that. It doesn't earn you any respect. It serves quite the opposite effect, in fact.