Yes, you can use Really Simple Security (Free) on WordPress Multisite. However, it is important to note that Really Simple Security Pro on Multisite environments does require the dedicated Multisite Pro version of the plugin, which is included with all of our Agency plans (25 sites and upwards).
Activating and managing Really Simple Security on WordPress Multisite is done within the Network Admin dashboard and the settings will apply to all (sub)sites on the network.
Please note that on WordPress Multisite, enforcing HTTPS/SSL with the Activate SSL button will enable SSL for all sites of your Multisite installation (even if a domain in the network does not have an SSL certificate).