What descartes says is that i think i am and you can't put a therefore in between because when he proves i think i am he hasn't proved that we can trust logic.
What descartes is say is that we can define humans as things that think.

For your skeptical argument i don't see the point because even if no one else besides me exist. We still move around in this world as if there existed other people that is why it's not such a interesting question on its own. Because if you believed in it i don't see the reason for posting anything here, because you would know that you would only be talking to yourself.
And if you say i could be dreaming... well everybody have tried to wake up from a dream to and know that it was only a dream.

And you can't even state that you don't exist because when you state that you use your memory, but your memory could be false so you don't have any basis for stating what you state.