Light advice for a 15 gallon tank with at 15 watt light

My friend gave me an old aquarium, it's 60L (15 gallons) and only has a 15
watt light in it. The surface is 60cm x 30cm. I just bought a Sylvania
GroLux light for it, based on the recommendation of my friend. We lined the
inside of the hood with aluminum foil to serve as a reflector. He also gave
me a some plants (2 different kinds) that he cut from his tank. I've been
reading the newsgroup archives and now I think my 15 watt bulb is not enough
for the plants. I don't really need the plants to grow very much, I just
need them to not die. My friend's grow very well, so he can always give me
some more from his tank if mine don't grow. Some questions:

1) Is 15 watt enough? I'm not sure what plants I have, I've searched and
looked at pictures but I can't identify them. I don't mind buying some low
light plants if I need to.

2) The hood only has one slot for one light. If I need more light would I
have to replace the hood? I see the retrofit stuff but the aquarium is very
old so I'm not sure how practical this would be. Might be cheaper to
replace the hood.

3) Should I add CO2? My friend has a homebuilt CO2 dispenser that seems to
work well and we can build me one for just a couple dollars. But I read
that CO2 doesn't make up for lack of light.

Thanks,
Michael