Problem with PostgreSQL and the geqo optimizer
I believe the settings you mentioned already ensure that geqo will be used, except that the values for geqo_pool_size and geqo_generations are so low that the results won't be good.

Apart from that, to get different plans, you should play with geqo_seed.

From the doc's section Generating Possible Plans with GEQO:


  As long as geqo_seed and the other GEQO parameters are kept fixed, the
  same plan will be generated for a given query (and other planner
  inputs such as statistics). To experiment with different search paths,
  try changing geqo_seed.


About geqo_seed:


  Controls the initial value of the random number generator used by GEQO
  to select random paths through the join order search space. The value
  can range from zero (the default) to one. Varying the value changes
  the set of join paths explored, and may result in a better or worse
  best path being found.