If it??s HT you are referring, a sub is always welcome. Not many floor-stands are up to that task.
It??s always advisable to stick to same series of a brand as this helps in tonal matching and there by helping achieve better sound stage as well as separation. Regarding mix-n-match, you may do that. The diamonds are accomplished speakers on their own. All you need is a good test CD/DVD/test-tones and a SPL meter ?? and of course a lot of patience as well!
I wonder why u might not want to consider the iQ5 package as a whole, I remember the sub in that series is pretty decent one - it looks good too!