Be firm, but don't be a jerk. Be reasonable, and honest - justify and explain. In writing if it helps. Just don't promise more than you can deliver, and be explicitly clear about the complexity of solutions.
If somebody gets in your face and calls you a ********, I want you to be nice. Ask him to walk. Be nice. If he won't walk, walk him. But be nice. If you can't walk him, one of the others will help you, and you'll both be nice. I want you to remember that it's a job. It's nothing personal.
Ahh... Road House continues to provide life's answers...
"To IBM, 'open' means there is a modicum of interoperability among some of their equipment." -- Harv Masterson