OK don't want/need Membership on my primary site, only on sub-sites (on demand)

I'm currently playing around with Membership on one of my MU installs which is set up as detailed here using the Global WPMU access (activates on all blogs in the install) - basically throw everything in 'mu-plugins' - BUT its not quite what I need and after reading a LOT of posts around here, I'm wondering of what I need IS in fact possible

BASICALLY - I don't need Membership enabled on all of my sites in the network and ESPECIALLY not on my main, admin-root blog/site - its really not needed here.

I only need the plugin to work on sub-sites where users want to run membership features. For users who are admins of their own sites and NOT super-admins and without any interaction of membership levels or users across sites. IS this possible? If so, how?

Do I need to install as a standard WP plugin and just get the user to 'enable' from their backend? From what I've read, I'm not sure it will show up because of the security features in the plugin around super-admin users?

Last note: I'm currently experimenting with the MP Blog Register plugin which fixes the issue of users registering for single-sites - it looks perfect, I just need clarity on Membership before I port this over to my live MU network.

tl;dr - I have membership set up network-wide, only need it on selected blogs in the network and definitely not on the root site