I don't know what elements can and cannot affect the Chungus spawn rates. The current version I'm using supposedly gives me the ability to place down the monolith in whatever biome, and it should have a chance to spawn within the 32-block radius. The thing is, I have yet to see a single Chungus near the monolith for over an hour. I'm not too sure what the reasoning is. I also checked the config file and tweaked it from 100 to 200 just in case, but still nothing. Maybe there are some other side factors involved?

If you are on the latest 1.2.x update then it should spawn in whatever biome in the overworld during nighttime. You can increase the radius of the monolith if you want or place more to make them spawn in a broader area. If you place it on a big platform in the ocean during nighttime, walk away or climb/fly high up, then come back, a lot of them should spawn.

Seeing as how you phrased it, I assume placing more wouldn't actually stack spawn rates too then? But I can place more than 1 at some distance away to increase the spawning radius?

Yes, the blocks only enable spawning so they don't stack, while placing more in a wider radius increases the spawn radius since they each only go about 32 blocks out.