Yes, it's perfectly fine to charge the battery overnight, and even leaving it on the charger until the morning won't make a noticeable difference in battery life. Some say you should take it off the charger right when it's full, but I've rarely done that and battery life hasn't been impacted.
As for a Siri alternative for Android, Google Now is probably your best bet. There are some things it can't do that Siri does, but I find Google Now is just a lot more powerful overall.