Since I have an 18" floor tom with my kit...at one point I actually thought about setting it up as a second, smaller kick drum with a bit of jury-rigging since it has no side legs/spurs (isn't there some add-on hardware to lets you do that?)...
...mainly because it was way off to the side, and it rarely got used.
But then I moved it to the opposite side...so the 18" isn't next to the 16" since it was being pushed too far behind the drummer, especially since there's a 14" in front of the 16". The 18" is now on one side and the 16" on the other, where it would normally be.
It actually works out pretty good that way, and balances out the kit nicely. The drummer can hit both the 16 and 18 by just spreading out both arms and back a bit instead of having to roll half way around.
This isn't a real high-end kit. It started out as a basic 5-piece that I got from a guy at work who's kid decided drums were not his cup of tea, so it was hardly played and almost new. Then one day I spotted a couple of toms on eBay that were the same model/color and two sizes I didn't have...so I bought them. After that, I found the 14" and then the 18" also on eBay...and now the last piece, the second bass drum.
I put all new heads on everything, bought a couple of additional snares in different sizes/styles, and then spent a lot of trial-n-error buying used cymbals on eBay. I must have bought at least 20 cymbals, and sold back 10, but I ended up with a nice set of Zildjian A and K cymbals, a couple of 13" Mastersound Hats A & K, and set of 12" Zildjian Recording hats...but I like the 13" Mastersound much better.
If I was a drummer...I probably would have bought some different things...but this is plenty of drum kit for my recording purposes...and it looks really cool, and it can sound pretty good (when I get it right)...I can't wait for the second bass drum to complete the layout.