That size tank would be good for a res up to 5 inches, however, he or she will eat any other inhabitants of the tank.. something you may not appreciate...
2 RES: Leo (f) and Ezra (m)
1 Russian Tortoise: Godzilla (m)
If you care about your fish, then no. Like megcornell said, they could become food. Over time, that tank will be too small for the needs of your turtle as well.
Depending on the size of those creatures, it sounds like it could become a heavy bioload rather quickly. I think a baby RES would do better in a tank all it's own.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-