A lot whould depend on the volume of the room/space being driven. However, It's my experience that you're NOT LIKELY to have "too much woofer", as long as you correctly match volume(s) with the main speakers.
For the record, most people have the subwoofers overpowering the main speaker(or mid/treble). But that said, often, there is not enough sub in SOME systems, or larger rooms, to properly handle the demanding bass dubties!
However, a lot depends on what volume levels you listen too. So for instance, it's possible you never push the volume level up much at all. Thus, some people can get away with a single, or marginal subwoofer situation.
In your case however, this will not likely be your concern.
It's been my expience(and I've had a lot), that you can potentially smooth out your bass in the room with a couple of woofers next to eachother in a room, and thus smoothing out some bass modes. So, 2 stacked, or 2 side-by-side. Each room is different, and results will varry.
If it were me, I would usually go with 2 stacked 15's on each side(4 total), over 2 18's. Experimentation and strong consideration of all the location options is recommended.
I would however avoid the often recommended "corner placement", as that ALWAYS LEADS to a "less than accurate" bass pressentation! YOur sound will be boomy, and "one-notey" in the bass, and you'll get "bombastic" sound even with instruments covering those frequencies covered by the woofers!...NOT NATURAL, OR WHAT WAS RECORDED!!!
The only time I'd place woofers in the corner myself, is if the sub was underpowered for the room, and needed some bass reinforcment that a corner placment would give. Even then, I'd be doing some SERIOUS EQ'ING of the woofer! So it is workable, but not as ideal. And in your case, you'll probably have more than enough woofers I suspect, so it shouldn't be an issue for corner placment.
Someone else in these posts also mentioned the Earthquake woofers. I must concur that the Earthquakes make stronger HT woofers with more "bight" and "weight" to the sound!
The Velo's are good in my oppinion for HT, but if I had to choose, I'd go with the Earthquakes in that regard I think....IMO.