Those POI spawners in the spawning.xml file aren't in use anymore. Those were for the outdoor zombies attached to each POI prior to the advent of sleepers. For increasing the number of sleepers try the spawners near the top of the gamestages.xml file, like this one here:
<group name="S_-Group_Generic_Zombie" emptyChance="0" decoyChance="0" decoy1hpChance="0">
<spawner name="SleeperGSList" count="5,6"/>
</group>
The count is minimum, maximum number of zombies; increase this to some larger numbers and it should work fine.
As far as outdoor zombies, the biome zombies you've found are the only thing going. There aren't any separate spawners for the city or town streets. The zombies in the towns and cities are the exact same biome zombies as the ones walking around the wilderness.