Best practice design ground and shielding
The digital ground and the analog ground should be connected, but by a
  small passage.


Yes this is correct. All you need between the two is a single point of reference. Best practice in most applications is a star ground, i.e. connect the digital ground plane to the power source's ground connection and the analog ground plane to the same physical point. The small passage should be capable of handling any current that could potentially flow through it.


  If the pcb have hole's for mounting to a metal chassis, should the skrew's then be grounded to the pcb analog/digital ground and to the chassis? Wouldn't that be a better protection?


This depends on the application. In some applications, such as underwater, the chassis should be electrically isolated from any charge storage element to reduce oxydization. If you want to electrically connect the chassis connect it to earth ground if the application allows.