I don't know what airline you work for, or what union represents you...but I wouldn't go into any disciplinary hearing without representation.
An email for a disciplinary hearing? Simply because you asked for a contract reference?
There's either more to the story than shared, or or the company is attempting to scare or intimidate you. Perhaps a little of both. If the scheduler "wrote you up" then the CP is probably obligated to "act" in some fashion.
RLA basically means "fly and grieve it". I don't see any issue with telling a scheduler you disagree and are going to file a grievance; does not sound like you refused anything so I'm not sure what they could tag you with other than possible insubordination (a VERY large catch-all).
Go in, be polite, don't assign blame, answer asked questions directly with no editorializing, take responsibility for what you said/did and admit you POSSIBLY could have handled the situation differently, and by all means if you rep tells you to keep your mouth shut DO IT.