I have CO2 on my 5.5-gallon tank as well as my 2.5-gallon ones. Just make sure that you maintain a KH of at least 5 German degrees (using baking soda, if need be) and you should be okay. For a 5-gallon tank, you could probably get along just fine using a small plastic soda bottle (I think Coke comes in 591 mL bottles). You could even use a 1-liter plastic bottle (I think tonic water and ginger ale commonly come in this size). I think the gelatin yeast method works best. I just had one (in the smaller coke bottle) last me almost three months. It was still bubbling but the gelatin had completely dissolved into liquid and I've had a new bottle set aside for about a week.
You really don't have to do anything special to improve the efficiency of the CO2 dissolution on such small-sized tanks. I had the exit end of the tubing going through an overturned second bottle cap to act as a bell diffuser (basically prolonging the contact of the bubbles with the water) but in one of the 2.5-gallon tanks, it worked *too* well and the pH took a nosedive. Granted, the KH was only about 3 degrees and there's no water current (aside from maybe heat convection). You could do this in the 5-gallon without too much worry, but just make sure to check the pH every now and then.
You sound a little unsure of how to go about setting up a DIY CO2 system, so you might want to read up on it a bit. If you do a Google search using the keywords "CO2 primer Cathy Hartland" you'll find a site that will explain how to put together a regular yeast CO2 system. You can scale down the recipe for your smaller tank. Maybe try the gelatin recipe later on. If you're left with more questions, there are a lot of people here that can help sort out the details. Good luck!
