I am posting for future readers:
The best solution is to make a very quick edit to the gamestages.xml file. Find the Feralhorde section:
<!-- XXXXXXXXXXXXXXXXXXXXXXXXXXXX start: auto generated feral horde gamestages XXXXXXXXXXXXXXXXXXXXXXXXXXXX -->
<spawner name="BloodMoonHorde">
<gamestage stage="1"><spawn group="feralHordeStageGS1" num="1" maxAlive="2" duration="1"/><spawn group="ZombiesNight" num="1" maxAlive="1"/></gamestage>
This means that if your gamestage is equal to 1 at the 7th night, you will get 1 zombie generated from the group feralHordeStageGS1, and one zombie from the group ZombiesNight.
<gamestage stage="188"><spawn group="feralHordeStageGS174" num="31" maxAlive="22" duration="3"/><spawn group="feralHordeStageGS181" num="31" maxAlive="22" duration="3"/><spawn group="feralHordeStageGS188" num="31" maxAlive="22" duration="3"/><spawn group="ZombiesNight" num="189" maxAlive="1"/></gamestage>
At gamestage equal (or great) to 188, you can expect 31 zombies from group feralHordeStageGS174 (only 22 alive at a time), 31 from group feralStageGS181 (another 22 alive at a time), 31 from the third group and so on.
In you want your early game to be more exciting (or even insane), all you have to do is delete/comment out the earlier entries for gamestages and change the left-over first entry to say gamestage stage = "1". That will force the hordes to start with that group and spawn rate until your GS goes beyond into the later groups.
I personally remove the stages after 1500 and make the last group the hardest setting (5000+). By the time I reach 1500, I dont want to wait any longer for the worst to come. Bring it on!