All of the Pokemon I've used so far are Kanto or Johto pokemon (gen 1 and 2) so either of those would work, as they would have trade with other regions.  For a primitive society, only having one region's pokemon makes more sense, because they don't have the pokeballs to facilitate trade of pokemon.

Once or twice a week would be fine.  I can post every day if I need to, but most people don't have the time for that kind of heavy posting.