Is this for a planted tank? Never heard of the 2.5W thing. If the tank is not planted, then you can use either the incandescents that come in the hood, or use a screw-in florescent. Wattage doesn't matter.
But if it's a planted tank, then you will need the Compact florescents, or the NO florescent hood. Incandescents do no good for plants. Also, if you go over 2w/g, you will need to inject CO2. But this doesn't apply to a 10G because of it's size. With 2 10W CF bulbs, you probably won't need CO2, but if you go up to the 20W bulbs, then you probably will need it.
But as for the 2.5W thing, never heard of it.