Anti-Spam on Multisite; General "Network Activate" question

I want to extend the WPMU anti-spam features to all sites on my network. Should I do this by using "Network Activate," or by adding those plugins to Pro Sites and making sure "Anyone" (even free sites) have access to them?

In general, what happens to a plugin when I "Network Activate" it? Does every site on the Multisite network inherit that plugin?

Thanks in advance for the info!