Moving from Wordpress to MU


We I a network of four independent wordpress sites as well as a vBulletin forum that I want to integrate into one MU installation. Been reading up on it and have started the process but can definitely use the help of the community. Right, so lets gets started

1) Whats the difference between enabling plug-ins site wide or specifically for each blog. For example, the four blogs I have at the moment are tech, games, lifestyle and movies. Now, if we take the All in One SEO plugin that I was using for each of the individual sites, do I enable it site wide or do I enable it separately on each of the blogs?

2) How do I share admins and editors between blogs?

3) Will any plug-in made for Wordpress work with MU?

4) How can I make more fields to appear during registration than just the email address and name- and make them manditory

I'm sure there are plenty more questions to come but this is a good start.