Full spectrum has to do with the color temperature of the bulbs.. that lamp comes with it's own special bulbs.
I don't think that you can just put a 13W bulb in a lamp designed for 27W. 13W is also a LOT of light for a 2.5 gallon tank.
You really don't want to go overboard with your light... the rule of thumb is 1-2 watts per gallon. In a heavily planted tank you can go higher, but above a certain point you have the balance that with injecting CO2 or you'll have an algae explosion I have a 5W fugeray fixture on my 4 gallon tank and I'm battling algae in that tank even with injected CO2.
I would stick to lower wattage range, which probably excludes CFL's on a tank that size... look into LED's, but again, be careful, because the wattage matters a lot... anything more than 5W is probably going to be too much.
You didn't mention if you are growing plants or not but for the most part full spectrum only matters for plants that like a lot of light - if you don't have plants or have low-light plants it's not as critical.