Sliders are wonderful turtles. I had a pair for about 12 years. They used to hiss at my father when he came into my room. Sliders will grow to their environment (i.e. bigger the tank bigger the turtle). I started mine off in a 10 gal, then moved to a 20 gal, then to a 70 gal. The turtles started off the size of a quarter and reached a final shell length of 9 inches. You need to stay on top of the cleaning (more so than with fish) as things will get stinky very fast. I was running a couple of large fluval cannister filtration units in the 70 gal and still had to do a good cleaning about once every 6 weeks.
Word of caution about leaving fish in with them -- the fish will soon be food when the turtles get larger. As a treat for my pair I used to get a couple of feeder gold fish and toss them in the tank. One gulp per turtle and the GF's were gone.
If they are not eating the turtle pellets, try a bit a raw beef, cut into very tiny pieces (about the size of the pellets). Clean out whatever they don't eat after 5 mins.
Best of luck.