Read up on Smart Objects. It looks like you have no idea as to how to create and use them.
Jut create a Smart Object from the layer containing whatever it image it is that you are "plugging into your template". But you do need to learn the application at its most basic levels.
Photoshop is a professional level application that makes no apologies for its very long and steep learning curve. You cannot learn Photoshop in a forum, one question at a time.
Or is it possible that you don't even have Photoshop proper but the stripped-down Photoshop Elements?"
If the latter is the case, you're in the wrong forum. This is not the Elements forum.
Here's the link to the forum you would want if you're working in Elements.:
If you do have Photoshop proper, please provide the exact version number of that application and of your OS.
(edited for clarification)
First make sure the original image is larger than you need. It is easier to remove pixels than to add non-existing pixels.
Place the image into the template.
In the filters menu, choose convert to smart object. (Make sure the image layer is the one selected before doing this step)
Now reduce the size of the image to what you need (ctrl-t or cmd-t) When satisfied click return/enter.
Because the image layer is a smart object, you can replace the inner image to another image and the scale will remain the same. You should find the replace image option in the layers>smart object menu.
If you need more help let us know.
Sorry I was unable to answer your post as expeditiously as you answered mine------I was getting assistance from another volunteer on my real issue (not you!).
As always, thank you so much for your provocative post. You never fail to amaze and inspire!