I believe that some canister filters come with a carbon pad. Just remove that and you will be fine. My eheim came with a carbon pad.
As for you right now, I wouldnt suggest using fertilizers and CO2 because your lighting is very low. Start CO2 and fertilizers when you can get your new hood whith lights.
IMO, eheims are better than fluvals. That fluval from petmart wont even come close to provide the filtration you need. I would get the eheim 2215. You might be able to get it at a cheaper price off ebay or aquabid.
One hagan CO2 wont be enough for a 55 gallon. You can buy at least two more hagans, make DIY CO2( a lot cheaper), or you can go presurized CO2 with costs about $150. Presurized is a lot more expensive but it is a lot less maintenance than DIY CO2 or hagan. For hagan and DIY co2 you will have to change the co2 mixture every to maintain a good CO2 level in your tank.