Hi Neil,
Adding the extra filter will always help when running a CO2 enriched tank. Gas injection produces better results than liquid carbon alone, but of course there's no rule against using both.
In the planted tank hobby there seems to be a tendency for people to assume they need massive lighting in order to be successful. This is a grand illusion, and the implementation of high energy lighting often results in disaster. There is plenty of light for plant growth, and you might even have more than you bargained for, so the last thing I would worry about is whether there is adequate lighting. Growing plants successfully is so much more dependent on other things. The best advice i can give you is to avoid being hypnotized by light and to instead, pay more attention to everything else.
Cheers,