If I have a rotation matrix and I multiple it with an identity matrix except that the x component of the identity matrix is -1 instead of 1, would I get anything meaningful and if so what?

You''ll mirror it about X-axis. So basically if you set the translation matrix of a triangle at (-20.0f, 0.0f, 10.0f) and then multiply the translation matrix with that x=-1 matrix, the triangle will be at (20.0f, 0.0f, 10.0f). Simple as that.