the problem with combining various speaker sizes for subs is that you're going to get cancellation most times which is self defeating, and you lose the benefits of each sub by combining them. his 18" is great for low bass if its crossed over low. the smaller subs would be great for midbass of the frequencies above the 18" but if they all do the same frequency range, the 18" will ruin any faster response from the small subs, and the small subs will muddy up the 18" for deeper bass. Also note he doesn't share air space in the enclosure between speakers which is one reason it works. What you really want to avoid is combbining different subs in the same air space. doing that will cause them to fight each other and cause damage.
Hmmm....when I first found this site I thought cool, but when you get into there's a lot of questionable info. Things like "no music below 40hz", "using sig gen at 20hz you won't feel or hear anything". Nobody can think tuning ported boxes to 45/60hz is a good idea, surely. Having Bassbox Pro doesn't make you a better person. As for the big box all those speakers do share the same airspace as there are "ports" between those chambers. Remember this site was initialy written several years ago and things move on.