Mongodb: Determining shard key strategy on compound index

I have a collection with 170 millions+ documents and it is only going to increase. The size of the collection is not that huge, currently around 70 GB. The collection has two fields **indexed on: {AgentId:1, PropertyId:1}**. Generally one imports a huge file(millions of documents) belonging to a particular **AgentId but the PropertyId(non numeric nullable) is mostly random unique value.** Currently I have **two shards with shard key based on {_id: hashed}**. But I am planning to **change the shard key to compound Index {AgentId:1, PropertyId:1}** because I think it will improve query performance( most of the queries are based on AgentId filter). Not sure whether one can have a nullable field in the shard key. If this is the case then app will make sure that the PropertyId is random no. **So looking to get a picture as to** 1. How the data will be distributed to shards during insertion and how the range of a chunks are calculated during insertion? 2. Since the PropertyId is random value. Does the compound key fits the definition of monotonically increasing value? I am a newbie to MongoDB. And wanted to know if I am on the right path?