One thing I really would like to see if the game will let you recruit or side with such creatures is that the narrative NOT assign a moral deliema to such events; or at the very least not assign 'Crazy Evil' morality for each and every creature you ally with.
Gaining such allies; like working with a group of Sylvans for example; should not always require that the player slaughter a Dalish clan or put babies on spikes to enlist their help.
Moral dilemma is this: The Disciples need Grey Warden blood to expand their numbers. That's all.
If you want to replenish their armies, you're going to have to get blood for them. One Grey Warden's life can quickly recruit many new Disciples, or you can convince the Grey Wardens to give up some blood willingly as part of a voluntary blood donation.
Disciples expand at a slower, but more sustainable, rate if you can get the Grey Wardens to agree to this scheme. Better hope you're a diplomancer.
Otherwise you could try to enact the Architect's "Kill 75% of the population" idea on a smaller scale and get even more Disciples at the expense of wiping out various population centers.
Or you could just wipe the Disciples out.
That's a good set of choices for various moral perspectives, or so I should think.