Firstly I am not a developer so I can add to much to the why’s etc. I will say one thing though.
It is not wise to create any design elements in 8.5 developer to run on a 6.5. This is because there are code and design elements in 8.5 that are not supported in 6.5.
You can try something to prove this. Create a design template in 8.5 designer then open it in 6.5 designer. When you try and make a change Notes will warm you that the template was saved in a more recent version…. and then warn you that you should not continue or design elements will be lost.
This will help for more information