A couple of MultiSite Questions

I am currently looking to change my current blog into a multi site network but I had a couple of quick questions that I hope can be easily answered before I do.

1. We would like to keep the main site but have subdomains that allow people who are only interested in that topic going there. Is it possible to have a main page that displays all topics over the various subdomains, without having to submit the story to two different networks.

2. Our current site contains some posts from two of the different subcategories that we would be making. Is it possible/easy to split these into their respective subdomains when we install switchover?

3. Does anyone have experience of working with Multisite with hosting on Streamline.net, are they okay for that sort of thing? Would we be better off transferring out hosting at the same time?