I just went digging through your monstergroups.json.
{ "monster" : "mon_failed_weapon", "freq" : 10, "cost_multiplier": 20 },
{ "monster" : "mon_zombie_failed_weapon", "freq" : 15, "cost_multiplier": 20 }
Cost_multilplier limits how many total monster can appear by subtracting from a pool (I’m not sure where the pool comes from or how much it is per area/horde/whatever), and freq determines how often the creature will get a chance to spawn.
As an example:
{ "monster" : "mon_zombie_brute_shocker", "freq" : 10, "cost_multiplier" : 20 },
A shocker brute will appear nearly as often as your failed weapons and less often than your zombie failed weapons.
{ "monster" : "mon_zombie_hulk", "freq" : 1, "cost_multiplier" : 50 },
A hulk has roughly 1/10th to 1/15th the chance to appear as one of your weapons. And it will cost the spawn pool 2.5 times as much to spawn one. Your weapons can thrash hordes of zombies, you may want to adjust them to be closer to the hulk, and maybe consider a weaker version of each for group_zombie to lessen their rampages through town in the early game. I consider them to be slightly less scary than a hulk when I’m playing, so they fit well in group_zombie_mid and as a reason to be nervous when visiting power stations.