I've run a pair of sealed 12" servos with planars before and the pairing blended together nicely. In that room, the whole rig ran better with a quad set, but in a room the size you describe (about half of what my bigger room was), I think the pair would be just about perfect.
One would work, but I'll admit that I'm a big fan of stereo subs, particularly if you plan on crossing that high up. If I were picking nits, crossed really high, I would be more worried about cabling the servo subs to get the tonal flavor to taste - to match up best with the Omega drivers. Don't worry at all about speed; there is nothing faster than these servo subs. Yes, I said it. Someone feel free to try to prove me wrong, short of maybe something like the old wall-sized Magnepans.
If you did go OB, I would set up a quad of them rather than just two drivers, as the efficiency mismatch with your speakers and the way the OB works might come up short with just two. The sealed subs should match up fine being a little generous with the gain dials on the plate amps. With your room size and moderate power into the Omegas, I would think you would still be good to go for headroom with a pair of sealed subs.
Once you get the subs in play, you might not even need to cross that high, depending on how the Omegas are tuned (haven't heard them myself). Don't think you need to cross that high, but don't worry about the servos not making music if you choose to run them higher up - I've gone that high and higher running the quad set sealed with no worries.
