Understanding DOF and variables.

Okay so up untill recently i thougth the only thing affecting your dof was the aperture size, however this seems not to be the case.

Example, f/1.8 on a small sensor might corrospond to something between f/5-10 on a FF. Why is this? Is there any formula for calculating this - relative to FF or 35mm.

Also, is there a constant relation between aperture, shutterspeed and ISO? If i shoot something at f/3.5 s/200 iso 100, will it capture the same exposure on a compact (:small sensor) and a dslr? Or are all these things.. relative variables?

I'm practicing dof photography, and playing around with settings, i can not make anything comparable to a straigth shot from a dSLR or a FF sensor ( http://www.stevehuffphoto.com/2012/05/11/my-one-hour-with-the-new-leica-monochrome-by-steve-huff/ ). I litteraly have to space a distance from subject to background at around 10 meters, full aperture at full telephoto to create anything similar. At wideangle, i can't create the effect satisfying.

That limits possibilities a lot. Granted, I'm still shooting on a finepix s5600 while figuring out what my next camera should be.

What should i be looking for on a camera to do this sort of photography? Must i have a dslr or can this be achieved from a compact camera?