Ruto was killed when they tried to seal Gannondorf in the Twilight Realm
No that's not true. After the events in OoT Link was sent back to his own time and because he informed Zelda in time none of these events happened so Ruto was never a Sage in CT. This has been stated by others before. So Ruto could not have been the Sage.
So the Monkey is not Saria. As you have noticed all the monkeys look the same. All have the same flower behind their ear. These were just monkeys living there and willing to help Link since he helped them to escape. We don't know saria is immortal. She is a Kokiri and they stay kids forever but none of us know if they are immortal. And the fact they don't leave the forest because they could get killed I assume they are far from immortal. It is okay to write down your theory but you must make sure you can provide evidence when you state facts like this