Wordpress Hosting Configuration Questions: Security and Load Balancing

I have a dedicated server; WHM/CPanel which is hosting multiple wordpress installations. I'm looking for advice on a couple of issues currently:

1. I don't have direct control over some of the installations. They belong to people paying for hosting and they are administrators. This means I can't make sure they are running current WP / vulnerable plugins. My question is how to insulate their accounts from others in the case of infection. I have heard of 1 installation being compromised and gaining access to all installations.

2. MySQL configuration. What is a good wait_timeout as well as simultaneous connection limit assuming several multisites with moderate traffic levels (100k or less page views per day). I'm also wondering how large I should make the tempdb and query time limits.

Any advice would be well appreciated.