Based on your drop checker readings, you can add more or take away co2. I think you said you have 2 2L bottles. I'm sure that is enough, but it may be too much. A drop checker will tell you if that is too much co2. If the drop checker says it's too much co2, you can go down to one bottle. If that's still too much, you can go down to a smaller bottle.
As far as diffusers, I can't say what would be best for you. In your small tanks, the venturi type reactor(power diffuser) would be big and take up a lot of space, therefore I wouldn't recommend it. I use the Hagen Ladder, which I picked up for $15 at my LFS. It works great for me, but others will say it doesn't work for a anything above a 20g. It's also kind of big for a 10g. I choose it because it's thin and sits well against the wall of my tank. I also enjoy watching the bubbles climb the ladder(just a personal thing).
A combination of airstone and bell type diffuser might work pretty well for you, but I can't say for sure.
