When you install the Search Service Application in the 2013 farm, you presumably install it on the Application server first. If you want to extend the Search Topology later on towards other servers (i.e. the QueryProcessingComponent and IndexPartition to the Front Ends) you have to think at which point in time you will do this. I have experienced that after content is being crawled, this is more difficult than before that moment.
I extend my Search Service Applications with the PowerShell script in the TechNet article found here.
Once I tried to accomplish this after the content had been crawled. This resulted in the following error:
Me and my colleague were able to solve this by adding a second index component to the topology and copy everything to a new topology.
So, when possible, extend a Search Topology before content has been crawled. That way it is much easier.