I've heard the same thing regarding large tanks and how they need less Watts
per gallon than a smaller tank. I've always wondered why this is the case?
1) If I have sufficient light on my 120 gal (@ 2 watts/gal) will the
ratio work for the 55 or will I need more/less light?
You will need a little more light. On larger tanks, you need fewer
watts per gallon to give the same amount of light.