Multisite Network Setup

I have a WP installation at a .com, and another installation in a subdirectory, let’s call it ___.com/multisite

I want to create a network in the subdirectory installation at my domain, so that my ___.com domain remains the site it is now (a separate WP installation) and the multisite ends up in my second installation in ___.com/multisite.

After I enable “Network Setup” in the ___.com/multisite WP instance, the tool page tells me that my Server Address will be the ___.com, not ___.com/multisite. If I install the network, will it ruin my first wordpress installation or will it only affect in .com/multisite?

Thanks for any help, and sorry about the long question…