First off your turtle will be fine eating guppies, fed them to mine all the time. The best stuff, however, is either home bred fish, or turtle food.
Frozen food that they enjoy eating is brine shrimp. It's extremly inexpensive compared to guppies, and they love it. The reason you don't want to feed them too many store bought feeders is because in the stores tanks, they use a lot of copper to keep the algae down, and that gets absorbed into the fish, and if your turtle eats the fish, thus getting the copper in it, too much can harm your turtle. I feed mine mainly the brine shrimp and every now and then ill throw in some ghost shrimp, or guppies for a treat.
Guppies will not reproduce by themselves. If you choose to breed guppies, good freakin luck. They are a pain in the butt to breed, just search the net you'll find out plans for breeding pools. As soon as the babies are born, they must be seperated from the parents to keep them from being eaten. Talk about motherly love, huh? lol...
Bloodworms i've been told can be fed to them, however I've never fed them to mine. Try baby crickets, however they do escape easily. when they grow older, you can feed them rosie red's, goldfish, however these also have high copper levels. Mealworms are a good treat, the only problem is that you have to drop them into the water and they will eat them one at a time, because the worms will drown. (i'm saying all this presuming that your RES is a hatchling...I have 2 RES hatchlings and one 1 yr old softshell). I feed my softy crayfish, I know for softy's at least that's thought to be a large part of their diet in the wild. If you really want to, you can every now and then feed them something like cooked chicken, beef, but in very small incraments. (if you cooked too much and you're afraid that it will go bad or something.) at younger ages, RES turtles are very carniverous, but later on in life their tastes will change to red-leaf romaine, brocolli, etc. Never feed them iceburg lettuce, however. They will eat this stuff now, in small incriments. HOpe it helped. -Chris