Personally I think you'll have a very tough time keeping a nice carpet with that amount of light, especially without co2. So I think hobby is right in the regards to the dilemma. Most carpeting plants need more intense light than just a typical stem to keep growing low and tight.
I've grown HC in a 9.5" tall tank with two 9watt cfl bulbs and I could clearly see a big different how the HC grew on top of a 5" slope (closer to the light) and at the bottom (further from the light)
Yeah, I hope to make the transition as healthy as possible for the first couple of weeks to hopefully have the HC in optimal health so when I go low on C02 and eventually cut down on it, it'll be ok. If not I'll still run CO2 but very low. Perhaps 1 bps or 0.5 bps to see how that goes. I think at that rate the CRS will still be ok so I guess its mainly trial and error. Too bad I don't remember the name of those who said something like "my tank grows HC just perfectly fine with medium light and no CO2". Would love to see how theirs turned out exactly, as in picture-wise : )