mixed swimming, converts

Question

Dear Rabbi: I wrote about the following two questions and since I have not seen a response, I'm asking again. 1. Is the issur on mixed swimming only for the reason of immodest dress or is there anmother reason? If there were no immodest dress, would actual mixed swimming be allowed? And at what age is a child obligated to observe this? 2. The Syrian community has a Takkana banning converts. Is this a halachically valid Takkana? Must one observe it? And , did the Rabbis in the community have authority to make such a Takkana that seems to go against whats specifically written in the Torah regarding how we should treat converts?