If you have some that are a custom size and you want them isolated from the others, you could use group2 or group3 just for those, and reference it from the code that shows in your header. As long as the banners reference group 2, and the location references group 2, it will only pull from that group for that section.
I do something similar on one of my forums at
www.ppfacorner.com If you refresh it several times, you'll see rotating banners from 3 different groups (each is a different size)