MarketPress help

I just set up a global marketpress installation on my multisite. I have a couple questions regarding how to complete my set-up.

1. I want to display all of the global products on a page on my main site. I have it so marketpress may be enabled on all of my subsites (so all of my members can have a store), and marketpress is activated on my main site. So, I have a store page on my main site, and on it are the Browse Products, Browse Categories, and Browse Tags shortcodes. That is great, but since I don't really want a store of my own, I just want a page that displays the global shortcodes and products of my members. Do I need both pages, or can I just disable the Store Page on my main site and Create a page containing the global shortcodes?

2. I want to be able to take a percentage of each item that each store sells. How do I set that up...I don't remember seeing that option in the network set-up menu.

3. Can I create global categories that cannot be changed by my members on their own sites, or do they choose their own categories?