It just depends on what kind of plants you want in this heavily planted fantasy tank of yours. You could easily have a bunch of nice crypts, java ferns, anubias, etc and do awesome. You have enough light though to incorperate plants with a little higher light needs if you want some diversity. I like ammania rotundifolia, maybe a lily, corkscrew val, alternanthera renekii. I would try some medium light plants and see how they fare. If they work out then great and if not stick to the lower light plants. And yes co2 would help. I think all plants benefit from the addition of co2. Although at 55 gallons that's a bit large for diy co2 but I guess you can do it. If it were me I would see if diy works for the size of your tank and how many 2 liters it will take to get ýour co2 levels where you need them. Then weigh the price of changing the yeast every couple weeks to how much a pressurzed system will cost you. It might be that diy + fish respiration might be enough for you, that's if you have a descently stocked tank. Good luck. Man that was a long post.