Difference between MEAN.io and Sails.js
The MEAN (Mongo, Express, Angular, Node) stack is just that: a pre-built stack consisting of those components. sails is also built on node.js and express, but it supports other frontend libraries beyond angular, and other databases beyond just mongo.
I think you wouldn't want to try to use them both; sails.js does everything mean.io does, but you get the additional flexibility of support for other UI tools and databases. sails.js supports over a dozen databases, including Mongo.
Some additional info on sails is in a similar StackOverflow question that I answered, here: What is the production-readiness of sails.js and meteor.js, and how to they compare to Django?