Passing parameters using Mashup parameters is an integral part to what you can do with Thingworx. You don't even need a ThingTemplate mashup, you can add parameters to a regular mashup.Depending on what you are looking to do, your parent Mashup would have your service and render the information in a list or grid etc. and then you use the Data Source's Selected row to pass in information into your contained mashup.
I already found a way to do it for Dynamic Services, passing the Thing Name as a string, but did not have success to pass an entity to the Thing Template Mashup.
For now I was able to do what I wanted with a regular mash but I was curious to understand how the Thing Template Mash works.