Multi DB - plugin clarification, please.

Hi all.

I have just successfully converted a vanilla install of WPMU to the 16 database option with multi DB as a test run for the site I'm actually working on. The actual site (though still running on my local test server) uses various plugins, and I'm reasonably confident I know how to edit db-config to add-global-table('tablename'); for these. What I wanted to check was if plugins added after the conversion will automatically add their global tables in the correct location?

Does it make more sense to install WPMU on the live server and do the conversion right away before installing themes and plugins, or to do the conversion on the testing server (including global tables required for plugins installed already) and then export the databases to the live server? (Can I even do that?) The usernames and passwords will be different on the testing and live servers, as will the IP addresses. I've never done this before, as it's my first WPMU install and I'm more used to single Wordpress installs where I can get the whole site ready on the local server and then edit the database connection details before uploading.

All advice appreciated.
Thanks
Janie