Can I add trackers to an existing torrent?

[Stuart]

I created my first torrent yesterday (woo!), but I wasn't sure which tracker to add, so I found a site with a ton of them and just copied one at random and put this in the torrent.

(here's the one I used: udp://public.popcorn-tracker.org:6969/announce)

Does it matter which tracker I used?

Can I change or add to trackers now the torrent is created? Would there be any point?

Thanks!