You could add the promo a number of ways. As you have suggested would work fine but you will need to override the start point for the music using one of the named markers.
You could also insert it as an advert, and leave your templates untouched. This would make it easier to change later without having to change each individual template.
Another option is to create the intro as part of your DVD template. This would save having to render it each time. You could set the DVD to play the intro upon disc insertion and then show a menu screen or set it to play before each viewing of their video.
We have a menu on our DVD with three options, My Skydive as default, Our Promo and a What's Next? video showing AFF. My Skydive changes for each customer but everything else is fixed and pre-rendered.
You will need a separate DVD Architect template for Blu-ray.
The default rendering settings for DVDs and Blu-rays are good for Australia.