Before I posted the questions I took a look for a solution to my problem. I think I saw this asnwer before and in tha threat, the guy ask you "Why this solves the problem". I also remeber that there is some functionality that is lost by doing what you suggest.
I'll test if my clients have ftp service indepedently one from the other after I do what you mentioned. But I would like to know if the DMZ has something to be in this problem.