I wouldn't worry about the CO2 levels in the tank. You should be alright. The one thing that I might be concerned with is the ph of the tank. I would keep a close eye on it. I beleive that the hagen system is designed for about 30 gallons, but if you have a heavily planted tank, then the plants will helpfully consume a good part of the CO2 being dispersed.
I am actually building a DIY reactor rather then using the stock difuser sold with the hagen kit. It is supposed to do a better job difusing the CO2 into the water. I have decent growth on my plants as of now, but I would like to make things more efficent with the reactor. If it works out well, I will let you know. You might be able to build a scaled down version for your tank that would optomise the CO2.
How is the lighting in your tank? for a 10 gal, I would shoot for about 35W to 65W and 6500K to 6700K. Between that and the CO2, you should be in good shape.