InstantSearch.js - Can I add several searchBoxes on the same page to search different types of content?

Yes, absolutely!

Following this guide on how to set up InstantSearch, there are no issues implementing several instances on the same page. Each of them can have their own searchBox widget and search a different index.

An example of how to implement this can be seen here:

https://codesandbox.io/s/multiple-is-intances-hmohd