I don't think you're being overly cautious. I'd personally recommend CO2 and ferts with 2x 54W HO T5s. I ran a medium light 55 gallon with 130W PC light, and the 108W HO T5 is MUCH brighter. We always had trouble with algae in the shop I worked at in the planted tanks with 108W HO T5.
If you add CO2 to a 55, I'd suggest going pressurized. The DIY method is cheaper, but a lot less effective in that size of a tank. You'd need a ton of 2 liters to make any appreciable difference. Kits like the Hagen Natural Plant system are pretty useless in a 55 too, even if you use 2 kits like I did. Pressurized would be the way to go.
If you don't want to mess with any of this, I'd recommend a 2x 65W PC light. This is pretty much the only time I'd suggest PCs over T5s.