you can install it in any directory in public_html, but not recommended to install in your zc directory. Yes, you should create a new database or you use your zc db if you can't add another (the installation script offers the option of a prefix to keep it separate from other tables that might have the same name).

There are a few small modifications to core zc files, well one in your case if you don't use attributes so you will only need to modify admin>orders. If you have never modified that file you can just use the one that comes with the zen cart connector.

There is a minimal configuration once the zc connector is installed but it's easy and like installing a new zc module.