How is stability ensured with bulk-loaded cargo?
You can simply tie down the cargo to prevent its movement. Most planes have nets that subdivide the bulk cargo compartment, so as to prevent movement of the baggage. For example, the following image shows the cargo hold of a B737 with dividers.



Image from airport-data.com Photographer:  Nick Taylor Photography

The ground operations manual of a carrier gives the following procedure for securing loads in A320:


  1.1.4.2.1. Load Securing A321 (bulk loaded)
  
  The divider and door net in each compartment must be closed at any time. Additional tie-down is normally not required except for individual items of load which by their nature, shape or density may constitute a hazard. They must be restrained, which can be achieved by filling the cargo hold or net
  section volumetrically, or by tie-down.
  
  ... Packages weighing more than 150kg shall be restraint or individually tied-down. Single packages should be tied-down. Tie-down of loads to aircraft structure is achieved by straps or nets connected to the tie-
  down points located on the cargo hold floor.