Installing wordpress on subdomain

I have a main wordpress site and i would like to add a Question and answer section to this existing site.Currently I am using a plugin for this purpose but would like to go for a separate theme for more functionality. I see there are some Q and A ready WP themes are available.

for example if my main wordpress site is http://www.domain.com, I would like to install a wordpress on a subdomain like http://www.forum.domain.com, and use any Q and A theme for this.
In this whatever the plugins (seo plugin, cache plugin etc) I have installed on my main WP site will available for the site added as subdomain ? or do i need to use seperate plugins for second website (In that all plugins I have install second time)

I have read about WP multisite feature. I am little bit confused abou this multisite and installing wordpress on subdomain. I will be using separate themes here for my two sites.

Please help me here to choose better option to use this new Q and A section for my site

Thanks in Advance
Anoop