plugins, themes and multisite security

Hi all. I’m brand-new to WordPress and I’m still getting a feel for how all the pieces fit together. My priorities are to use BuddyPress, bbpress, and have a way to sell products, for example with WooCommerce. It’s been more than a little frustrating finding a theme that I like, that works with all three. As far as I can tell, none of the theme frameworks have built-in support for these plug-ins. Is the only solution to build a custom theme that works with all three, or have I missed something?

I’d also like to offer my clients a blog, using multisite. If my website is on the same domain as my clients’ sites, does that present any security issues to my store, such as XSS?

Thanks!
Paul