This uses different parts of the Mission Dreadnought squads for each chapter to make the more distinctive. It would well as a intermediate step up in a campaign as well, like four missions with the standard squads, then four missions with these rules then the last four with the four full Mission Dreadnought squads.
Blood Angels: Seven man squad, up to five bolters available. So basically 1-3 heavy weapons/3-5 bolters.
Imperial Fists: Five man squad with the tarantula available. You loose a heavy weapon but the commander already has a plasmagun.
Ultramarines: Seven man squad with the three extra heavy weapons available.