I think if you are going to do this and make up characters you have to start with the base hero or model stats and add to it. However, for anything that is added, you have to justify it. For example lets make a marine strength 5. Well why would he get that, because I want him to, bad answer, because he has been in so many battles he knows which part to hit that would cause the most damage, good fluffy answer.
I used to roll the dice, Feel the fear in my enemy's eyes
I'm afraid there's no authoritative source for thinking you're an asshole