(potentialspawn.json)

[{
"dimension": 0,
"block": "minecraft:grass,minecraft:dirt,minecraft:snow,minecraft:ice,minecraft:sand,minecraft:sandstone,biomesoplenty:grass,biomesoplenty:dirt,biomesoplenty:white_sand,biomesoplenty:white_sandstone,biomesoplenty:dried_sand,biomesoplenty:hard_ice",
"mobs": [{
"mob": "thebetweenlands.siltcrab,thebetweenlands.gecko,thebetweenlands.dragonfly,thebetweenlands.firefly,TwilightForest.Wild Deer,TwilightForest.Bighorn Sheep,TwilightForest.Wild Boar,TwilightForest.Raven,TwilightForest.Penguin,twilightforest.bunny,twilightforest.squirrel,twilightforest.tiny_bird",
"weight": 10,
"groupcountmin": 1,
"groupcountmax": 5
}]
}]

(spawn.json)

[
{
"dimension": 0,
"mob": "thebetweenlands.siltcrab",
"biometype": "WARM,HOT",
"block": "minecraft:sand,minecraft:sandstone,biomesoplenty:white_sand,biomesoplenty:white_sandstone,biomesoplenty:dried_sand",
"result": "allow"
},
{
"dimension": 0,
"mob": "thebetweenlands.gecko,thebetweenlands.dragonfly,thebetweenlands.firefly,TwilightForest.Wild Deer,TwilightForest.Bighorn Sheep,TwilightForest.Wild Boar,TwilightForest.Raven,twilightforest.bunny,twilightforest.squirrel,twilightforest.tiny_bird",
"biometype": "WARM,HOT",
"block": "minecraft:grass,minecraft:dirt,biomesoplenty:grass,biomesoplenty:dirt",
"result": "allow"
},
{
"dimension": 0,
"mob": "TwilightForest.Penguin",
"biometype": "COLD,SNOWY",
"block": "minecraft:grass,minecraft:dirt,minecraft:snow,minecraft:ice,biomesoplenty:grass,biomesoplenty:dirt,biomesoplenty:hard_ice",
"result": "allow"
},
{
"dimension": 0,
"mob": "thebetweenlands.siltcrab,thebetweenlands.gecko,thebetweenlands.dragonfly,thebetweenlands.firefly,TwilightForest.Wild Deer,TwilightForest.Bighorn Sheep,TwilightForest.Wild Boar,TwilightForest.Raven,TwilightForest.Penguin,twilightforest.bunny,twilightforest.squirrel,twilightforest.tiny_bird
"result": "deny"
},
{
"dimension": 0,
"minlight": 4,
"mob": "minecraft:zombie,minecraft:skeleton,minecraft:creeper,minecraft:spider,minecraft:cave_spider",
"result": "deny"
}
]

I followed the example on getting blazes to spawn in the overworld and figured I could do the same for mod-added mobs like above but it doesn't seem to be working. Any help is appreciated!

It may be that In Control isn't capable with a given mod. For instance: In Control seems to have no ability to make Aether creatures spawn outside the aether (I haven't achieved this with Custom Mob Spawner either).

Some mods work with In Control, some don't. I suspect it has to do with where In Control falls in its management of spawning. I can't say for certain though.