Which url is best for my multisite setup?

I have a question about a wordpress multisite network setup. Here’s what I want to achieve. One central membership site which will be running Pro Sites, Coursepress, Tutorials etc which will use Protected Content to control access. This will be a multisite network site on http://www.mydomain.com

I currently have two websites which will feed customers into http://www.mydomain.com to purchase through Pro Sites i.e.: http://www.saleswebsite1.com and http://www.saleswebsite2.com ,then each sub-site will be on subdomains of this i.e: http://www.subdomain.mydomain.com

My question is which url should I use, does Domain Mapping allow me to point any url to any site or sub-site?

I am struggling to understand the role the main network site plays with multisite. Is it best practice to use the main url ( http://www.mydomain.com ) as the main membership / pro sites home base? Can I use domain mapping to point any other url e.g.: http://www.myotherdomain.com to the main network site http://www.mydomain.com ?

Thanks! It just seems I should get this first step right before stepping into all this development work.