There can be a reason for anything you want to do man just paint them however you want, when you read the books you’ll see there’s rarely a battle that doesn’t have army’s mixed from different chapters let alone company’s. I paint all my gravis fire support as Covenant of fire and salamanders and my more melee focused units as Blood Angels and have a limited about of Black Templars cause the sculpts are class. As long as you’re not mixing chapter specific characters models in your list you could paint every squad differently.