Actually, the Tauren and Trolls aren't conflicting additions to the Horde. They carry a lot of the same values and revere the same things (the Spirits) as the orcs, and are very close to them due to gratitude, friendship, and respect. They are inter-knit so closely that you can basically treat the three races as a single one when making assumptions about their choices. At least while Thrall is in charge, anyways.
It's the addition of the Forsaken that really messes things up, because a significant portion of the Forsaken as a collective are actively contemptuous of the living. And the leader's majordomo is a demon. Straight up, Sylvanas' primary advisor and enforcer is a demon ultimately loyal to the Burning Legion. And the Forsaken must, by necessity, use necromancy to increase their numbers; necromancy, which is just as corruptive and evil as fel magic.
If the Forsaken are actually a member-race of the Horde when we first meet them, there is no way we'd have peace with them. Our background would literally be screaming inside the back of our minds at all times to kill the Forsaken to the very last individual.
Blood Elves wouldn't be much of an issue, I think. The only difference between a Blood Elf and a High Elf, currently, is whether or not they're loyal to Kael'thas first, or the Alliance first. We're likely to make some headway with the High Elves directly, with the Alliance stuff coming up soon. For the Blood Elves though, basically as long as we catch Illidan and all them as soon as they hit Outland, we can basically go with the approach of "Stop! You are being tricked down the path of corruption by the demons! Let us help you be free of their deceit and evil before it is too late! We can help you!"