For your use case, probably the best would be one index per 'queue_name', or even one DB object (so with _id index only) per queue.
CodernityDB Database is kind of container for indexes (or indices if you prefer that name). It has one mandatory _id index. Everything have to be there. Then you can add secondary indexes, that will allow you to separate different things from each other (but they will still be in _id index). Please take look at: http://labs.codernity.com/codernitydb/database_indexes.html#tables-collections it should explain this to you.
But if your message (i think so) is not related at all to other message, it might be good for you to create database per queue, it will have less overhead, and then you can use _id index directly.