Here are the guidelines on supplements:
http://www.redearslider.com/supplements.html
If your turtles are healthy and getting a good diet and basking under good light, they won't need much in the way of supplments, so be sparing. They won't need it every day.
On the cuttlebone - YES they are old enough. Cuttlebones are great. Not only are they an excellent source of calcium, they also keep your turtles' beaks trim. Be sure and pry off the hard, plastic-like backing. You can break the cuttlebone into pieces and leave a piece or two in the tank for your turtles to snack on when they feel like it. Or you can fasten a piece of cuttleone to the side of the tank with a veggie clip. Makes it easier for them to eat.
And yes, they are old enough for guppies.
On the light bulb, here's some info on a specific bulb and supplier:
http://www.reptilesupply.com/product.ph ... ts_id=1817
If you get your bulb from a pet store, here are some do's and don't's. Be sure the packaging specifies the amount of UVB. The amount should be at least 5% but less than 10%. It must be fluorescent. If you get the Hagen Exo Terra bulb get the 5.0 26 watt one. I'd avoid an ESU. A reptisun 5.0 by
zoo med is ok as long as you don't get it too close to the turtle - it's strong and the light could hurt your turtles' eyes if it's too close. Don't get the reptisun 10.0.