don't know about slavery (it was allowed but there were laws regarding it), but here's a quote from the prophet's last sermon that should answer your second question
"All mankind is from Adam and Eve, an Arab has no superiority over a non-Arab nor a non-Arab has any superiority over an Arab; also a white has no superiority over a black nor a black has any superiority over a white - except by piety and good action. Learn that every Muslim is a brother to every Muslim and that the Muslims constitute one brotherhood."