I believe that the Silverfish mob in vanilla Minecraft should be able to spawn in specific places within Middle-Earth. They are in fact a real life insect.
In the mod, they should be able to spawn within dark places and ruins. Specifically they should be able to spawn within Infested Blocks in structures that include:
Ancient Harad Fortresses.
Ruined Beacon Towers.
Gondor Ruins.
Ruined Harnennor Settlements.
Ruined High Elven Turrets.
Mumak Skeletons.
Numenorean Ruins.
Rhudaur Castles.
Ruined Dwarven Towers.
Ruined Dunedain Towers.
Ruined Gondor Towers.
Ruined Houses.
Ruined Wood-elven Towers.
Small Stone Ruins.
Stone Ruins.
Underwater Elven Ruins.
Termite Mounds.
Ruined Overgrown Roads.
Barrows.
Boulders.
Orc Dungeons.
Troll Hoards.
Taurethrim Pyramids.
The Pits.