Using both your suggestions I got spawning working. It looks like I needed an EntitySpawnPlacement as well. I didn't see any spawns until I added the placement. Thanks for all your help!
private void registerEntitySpawn(EntityType<? extends EntityLiving> type, Biome[] biomes, int spawnProb, int minSpawn, int maxSpawn) {
EntitySpawnPlacementRegistry.register(type, EntitySpawnPlacementRegistry.SpawnPlacementType.ON_GROUND,
Heightmap.Type.MOTION_BLOCKING_NO_LEAVES, null);
for (Biome bgb : biomes) {
Biome biome = ForgeRegistries.BIOMES.getValue(bgb.getRegistryName());
if (biome != null) {
biome.getSpawns(EnumCreatureType.MONSTER).clear();
biome.getSpawns(EnumCreatureType.MONSTER).add(new Biome.SpawnListEntry(type, spawnProb, minSpawn, maxSpawn));
}
}
}