You need to be careful with Document resolution and image files resolutions and. Photoshop does some scaling when you use Copy and Past and Place and may additionally scale the smart object layer if it doesn't fit within the documents canvas.
One of the reasons I posted my Photo Collage Toolkit is the my Template use Alpha channels to Map Image locations. The Alpha channels are used by my scripts for several things: They use to scale placed images to fill the image location. They are used to position images and they are used also mask off any excess image what the image aspect ratio differs from the location area aspect ratio. Alpha channels can also be any shape.
Hers is an example of a filled template and a video of the template being populated in a fully automated process.
