rtri += 0.1f; rquad += 0.1f; I'm not sure how GLUT handles loops because the above two lines confuse me. Each time the loop runs should the two figures not rotate faster and faster each iteration, because they don't.

the figures rotate at a constant speed. rtri and rquad represent the total angular rotation of the polygons not their angular velocity

Ok, after reading this a few times and running the example program I'am beginning to understand rotation. However, the coordinate system that OpenGL uses how much is 1.0f on the monitor.

That depends on, the view matrix, the resolution, any transfotmations made, the xyz value, in witch vector is the 1.0f pointing and so on.

It doesn't really matter, use whatever looks best, personaly i assume that 1.0f=1meter.
That gives me a scale that is both SI compliant and z-buffer friendly.