If you're referring to limiting speed via your torrent app, you should be able to define your maximum upload speed and your maximum download speed separately. If your specific app doesn't support this, you should look for another torrent client that does.
You can certainly use ALTQ to limit bandwidth usage. However, limiting download speed is silly. Consider - by the time your box knows that the download speed limit has been reached, the packets that exceed the speed are already at your box and have used your downstream bandwidth. Therefore, the firewall will happily drop them, but the bandwidth has already been used. The protocol is most likely TCP, so they'll be re-transmitted anyway, using still more bandwidth. This is counterproductive to what you're trying to accomplish.
If you do decide to use ALTQ (which is a very effective way of limiting upstream traffic) you'll need to compile a custom kernel because the generic kernel that comes with FreeBSD does not include support for it. You can have as many rules to shape the traffic as you need (within reason). I currently have about half a dozen rules active on my mail server so that when somebody e-mails a 10 meg video to 20 people, the mail server still works instead of doing nothing but trying to deliver all the huge messages to their destinations. Check the handbook for details and examples on how to create a rule set appropriate for your purpose.
