Hi,
I'm one of the developers of the Foxx and JS driver teams at ArangoDB.
I'm not aware of any direct integration of Foxx in sails, but as Foxx lets you implement your own HTTP API on top of ArangoDB it shouldn't be different from using any other HTTP API within sails.
It is possible to use ArangoDB purely as a REST database without using Foxx at all.
It is also possible to implement your own domain-specific HTTP API on top of ArangoDB using Foxx and not make any use of the built-in REST API at all.
Which approach works best depends on your project's needs and how comfortable you are with offloading certain parts of your app into the database using Foxx.
I don't have any experience with sails.js personally but I don't see any reason you shouldn't be able to use Foxx and sails in the same project.
Hope that helps.
Cheers,
Alan