The Dark Angels refuse to fight alongside non-humans except under the most dire of circumstances. Because of this, unless you are playing a special scenario or game that is part of a campaign, then the Dark Angels will not fight as part of an army containing detachments that include any of the following troops: Ogryns, Ratlings, any non-human race. In addition, a Dark Angels army may not include any allies from the Heroes of the Imperium list unless you are playing a special scenario, though in this case it is because they don't want the outsiders 'sticking their noses' into the Dark Angels' private business.
Errata — Chapter Approved: Codex: Dark Angels Q&A (v2.0): Although the Assassins Codex would suggest Assassins can be taken in a Dark Angels army, the Dark Angels Codex takes precedence: you can only use Assassins in a Dark Angels army when playing a scenario of your own devising or as part of a campaign.