TiZ, does the room giving you the Cacodemon problem contain a map objective like a key or a keyed door, or the exit?
One thing I've noticed about the monster control settings is that, even if you set a particular monster as "scarce", sometimes OBLIGE 7.50 will still place these monsters as guardians of map objectives. For example, I particularly hate Arch-Viles, but even with them set as scarce, I can still expect to find them in several maps guarding the exit, sometimes even in groups of 3 or 4. I'm fine with this though, since it makes sense to me that map objectives should have tougher monsters guarding them.
Also, you mention having the monster quantity set to "more" which, I believe, does indeed generate larger groups of the same types of monsters, but I did not see mention of what setting you are using for monster variety. I think if that is left at the default, you will indeed see rooms filled with a single type of monster.
I don't have anything to do with the development of OBLIGE, of course, but I've been playing with it a lot lately so hopefully my observations can be of some help.