If I promisify some code with bluebird, can I use it with Mongoose's built in promises? Or will promises from different libraries explode when they touch?
Well promises are suppose to be compatible with one another due to adhering to rules. Should be A-OK. Bluebird is just, at the moment, the most performant Promise library, and they seem to be building their API as close to what native Promises will be like. You could really use any promise library and it wouldn't matter much, perhaps use the one that mongoose uses if it's an issue.