Yes. Hover over the tweet and the 'reply' button will appear for you to click on. Alternatively, you can just type their username (@username) before your message as the reply button just puts the username into the message automatically for you.
There is no problem with reply to an old tweet by someone famous.
It's like adding a comment on a blog post. You're replying with your thoughts about that tweet. Then the person who published the first tweet can see what you said and maybe reply back to you too.
Tweet is like status updates or reply to posts on facebook!
Compose a tweet and mention them in it.
Send them a tweet and hope that they will reply/follow you.
Try tweeting at them they may reply.
By clicking on 'reply' or starting a tweet from scratch and starting it with their username (@username).
a retweet cant be edit a quote can be edit
Yes you can still tweet them and they will still see the tweet in their mentions.
You can't automatically add it as such. On the Twitter app, you can start typing a hashtag and it will give you suggestions of previous hashtags from around Twitter.
Yes, you can tweet anyone.
You can tweet him on twitter, but there's not always a chance that he will reply.