Doesn't Diluted or Thin Paint Lack Intensity of Color?
Fluid acrylics and acrylic inks are manufactured to have intense color when dried. If you dilute a heavy body paint with an acrylic medium, you're not diluting the color because the medium is colorless; it only changes the viscosity (liquidness) of the paint.
Does this Technique Work with a Flat Canvas?
If you put a canvas down flat, gravity will have less of a pull on the paint so it won't flow so dramatically across the surface. Rather it'll spread only a little way, giving you a greater degree of control. How far it'll spread will depend on how much paint you pour out, how fluid the paint is, and how wet the other paint on the canvas is.
Does the Pouring Technique Work for Oil Paints?
Pouring paint will work for any paint provided it's fluid or liquid. The disadvantage with oil paints is that it takes so long to dry, so you'll either have to do the painting over quite some time or do it entirely wet-on-wet.
Is This Technique Suitable Only for Big Canvases?
Not at all, it'll work on any size canvas. A large canvas will require more paint but give a little more room for 'accidents'. A small canvas will use less paint, but you'll likely want to try to be a bit more precise about where you pour the paint and try to spread it so you don't have every color going over the whole surface. Experiment and you'll find out.