As per james comment Co2 helps with these plants, are you adding any fertz - I have attached two shortcut to these plants requirements, have a read and you will see that Co2 (as per james comment) and fertz are required for these plants.
Graeme and I spoke to Lars at Tropica and he puts it down to a lack of Trace elements, all of which can be found in Tropica Plant Nutrition, the plus variety has the added benefit of N+P so i'd go for that.
You'd be better off using Excel or Easycarbo rather than Yeast CO2 IMO. When I used Yeast generated CO2 on my 25l nano I had continual algae growth until I used both CO2 and EasyCarbo. As it turns out the EasyCarbo or Excel will do as good a job on it's own. George has done some excellent scapes with only these liquid carbon supplements.
Yes it's possible as long as you don't use too much light. If carpet plants such as these are important to you then life would be much easier with a pressurized gas cylinder system. Liquid carbon is not as effective as as gas system so your margin of error is slimmer. Depending on your tank size and filtration, it could be very expensive to do daily dosing of liquid carbon. Depending on where you live you may not have access to certain equipment though, I realize that. Are you in UK?