1. ClassiPress is a stand alone theme, so is not made to be integrated into an existing theme.
2. Yes, the easiest option to use ClassiPress with your existing theme would be to install another instance of WordPress in a subdirectory and create links between the two e.g.
www.yourdomain.com <== Your current site
www.yourdomain.com/classifieds <== Your ClassiPress installation
3. No, because they would be installed on totally different WordPress installations.
4. If you did as mentioned in point 2, then no, your original theme wouldn't be effected at all. The ClassiPress installation though would have the default look at the theme itself, so if you want it to resemble your existing theme you would need to do some customizations to achieve this.