OK, sorry to butt in here, but, just to pick hairs, if you did not wish the fact that your horses were rubbing their tails to be taken into consideration it might have been best not to put that info in, in the first place?
Basically, if you only want comments on a specific thing it is best just to put that specific thing or to state that you know what is causing it and you are taking this course of action so do not need comments on it, thank you!
I know it sounds a bit crazy but I do believe you are being too sensitive, that there are many forms of worm infestation and your worming programme may well have missed something, so a very mild suggestion that this could be a cause of your base problem (after all stalling them will not prevent them from rubbing) could have saved you a lot of time and bother.
This is the way this forum works, nothing disparaging is meant by the comment, it is just someone trying to be helpful.
I see lots of really good ideas here, for stalls, but personally, I like a situation that allows animals their privacy as well as access to companionship.