I am in the process of starting a client/reseller hosting

I am in the process of starting a client/reseller hosting business. Providing package offerings based on design and features. That said, as I read through the MultiSite section, it seems many do this for unrelated businesses. So, I am trying to wrap my head around the benefits and risks. Wouldn’t placing ALL clients in one WP MultiSite be placing all eggs in one basket?

How does one deal with subdomains of a site, i.e. site10.com and test.site10.com within a MultiSite? I am wanting to provide a test site for them to see site upgrades prior to making it live.

I’m new to offering these services and digging into as much reading as I can to see how WPMU can help make the learning curve smoother. Any best practices, posts, manuals, etc would be very appreciated. Thank you!

jw