The old WPG guidelines really only apply to the older T8 and CF fixtures that aren't used nearly as much these days. A T5HO fixture with a decent reflector behind the bulbs can easily produce almost 2x the usable light at substrate level as those older fixtures.
Have you already purchased the Odyssea? If so, you can probably work with that and not need CO2, if you take other steps to reduce the light- such as suspending the fixture higher up off the tank or using something to obstruct some of the light, such as mesh, floating plants, etc.
If you haven't purchased the fixture yet, and want the option of possibly going high tech with your tank down the road, then I'd encourage you to consider three bulb fixtures with the bulbs on different switches (giving you the option of running any combo of 1, 2, or 3 bulbs at a time). Not sure what all is on the market with three bulbs ATM, but I know Catalina will custom make one for you for a very reasonable price (I happen to really like these fixtures, personally... I have no affiliation with the company, however lol):
http://www.catalinaaquarium.com/store/index.php?cPath=71_136&&page=1
I'm not a big fan of Odyssea brand... but I do hear they've gotten better quality than they were back when the ballasts used to constanty go on the fritz?
