Multisite and content aggregation

I’m about to shift my whole site over to WordPress and want to know if my planned structure is possible.

Let’s say I’m running an umbrella site called (for example) bicycles.com. I’ll create all sorts of content in the main site under headings of say mountain bikes, road racers and BMX. People can come to bicycles.com and see a heap of content, all about bicycles.

What I’d like to be able to do is automatically aggregate these stories down into bicycles.com/mountainbikes, bicycles.com/roadracers and bicycles.com/bmx so that people can access specific sites quickly, and more importantly I can target advertising to these specific sectors of the market.

Each of these sites will visually be almost the same as the main site, but will have subtle differences like a change in colour.

Obviously, I’d like a common user login, and have comments posted in all areas update others.

So my questions:

1. Is this kind of thing doable at all? And if so, is multisite the best way to go about it?

2. Can a single theme be used, but the key differences be handled via a separate CSS specific to the subdirectory?

Thanks in advance.