I used those two sets to build my Viking warband. I used a very simple system to differentiate them. Dark Age Warriors box set bodies do not have armor. The Hirdmen bodies do. So, my levy came from Dark Age Warriors, so no body armor. They got bows only, and mostly heads without helmets. For my warriors (since they are better armored than levy), I used the Dark Age Warriors set bodies and gave them a hand weapon and a shield and heads with helmets (for the most part). For my hearthguard, they got bodies from the Hirdmen box (they have chainmail), a hand weapon, a shield, and a helmeted head.
So, i can look at any model and tell what it is at a glance. Chan mail & shield? that is a hearth guard. Shield but no chain mail? warrior. Bow and no armor or shield, that is a levy.
I also made a unit of berserkers by using the Dark Age Warrior body and gave them two axes each. they look a little silly, but easy to tell that they are special.
Here is the only problem you will run into: The Dark Age Warrior box only comes with 8 bows and the hirdmen box comes with none. So, you aren't going to have enough bows to make a 12 man levy.