You need the Current reference files 1st
Extract the Rebellion folder to your desktop or where ever your work space is.
Create a folder inside
C:\Users\[username]\Documents\My Games\Ironclad Games\Sins of a Solar Empire Rebellion\Mods-Rebellion v1.52 Dev
The folder name will be your mod name
Inside your new mod folder create another folder called GameInfo (Case sensitive)
copy the files you want to edit from the current reference files to your mod and edit with NotePad++
Test using the dev exe located in the install directory usually
C:\Program Files (x86)\Steam\steamapps\common\Sins of a Solar Empire Rebellion\Sins of a Solar Empire Rebellion Dev.exe
I would make a shortcut to desktop of the Dev.exe
test your changes often as the dev exe will show many errors you will not understand
And yes those are the .entity files you want to edit.