Privacy aspects of Safari extensions
In general, extensions should only be installed from trusted sources. See the popup that appears when you attempt to install an extension below. Unlike Chrome's extension API, Safari does not require that extensions request specific rights from the browser that the user must in turn grant when installing the extension, or specify a set of domains that the extension is allowed to operate on.

The developer of a Safari extension must, however, be part of Apple's Developer Program, and must obtain a signed security certificate from Apple for an extension to load in a user's browser. Unlike iOS or Mac App Store apps, though, this does not mean that Apple reviews extensions that are not hosted in the Safari Extension Gallery on apple.com.

The second part of your question is out of scope for this site, as it requires interpretation of Google's TOS and Privacy Policy (as linked from the page you linked).