Torrents in and of themselves are not illegal, for example Ubuntu uses them as their main form of distribution and a few bands have even released their records via torrents (ie Nine Inch Nails and Radiohead) so I think as long as you keep on the subject of the torrent client and not anything naughty you get with it you should be fine.
As for a good port, I've never paid much attention (verizon seems pretty good about not port blocking) but there is a full list of ports and there common uses over here
[link]