Can anyone share some general information concerning shooting 15/30 second
commercials? I've learned a lot on this site about standard settings for features, etc.
Are there any set guidelines to follow before the TV stations to air commercials?

As far as any guidelines go, you may want to contact your local cable company, or Tv stations that they may air on. They can probably help you there. Camera settings are another thing. In most of my cases I'm still shooting them 4/3 60i for local commercials.

I've shot two commercials with my XL2. There really wasn't anything special to do. I did one in 4:3 and one in 16:9, used 60i for them both I believe.