How I can allow users to have restricted-read permission only on certain documents or pages

I have the following inside my publishing site:-

1. 5 enterprise Wikis sub sites.
2. For each sub-site I have defined unique security groups on who can read, contribute, have full control on each such sub site.

But currently I need to grant certain authenticated customers the permsion to only view specific Wiki articles pages, so I follow these steps:-

1. Go to the desired wiki page.
2. Click on Edit.
3. Go to “Page permission”

But I was redirected to the subsite permission groups , so if I define a new restricted read group and I assign the external customer to it, then the external customer will have restricted-read permission on all the sub site articles which I want to avoid, as I need him to only read specific Wiki page.

Second problem I am facing is that to be able to create new group on the sub site or page level, I have to stop inheriting permissions from parent site. So why I cannot create new group at the sub site level or at the document level unless I stop inheriting permissions from parent site?