should I use 1 wpmu installation or do I need more?

Hi, Im testing to run a wpmu site for 1 community,
with your plugins multi-domain and domain mapping I know
I can run thousands.

But my question is that they would all have to register thru
the main site right?
so If the main site is about Coaching and I want to have
domains about Food so people create food blogs, they would
all have to go to the same main registration?

Also with your plugins that show global tags, global categories
would they show all mixed up?

So in this case should I then have 2 wpmu installations one for
coaching community and separate for food?

In the Multi-Domain plugin I have to point all subdomains to the
main registration with *.fooddomain.com but would I still be able to have a homepage for that page? I mean fooddomain.com/index.html
where people can read what is about, or everyone visiting that domain would have to go and be pointed to my main wpmu installation?

Thanks I hope I explain right and made questions clear

Cheers
Victor