It's weird some people are seeing Armus in the Ba'ul. Aside from the fact that both of them may be covered in some kind of black pitch or slime, I'm really not seeing the resemblance.
Armus was the black tar creature. It could become amorphous, be a lake of tar, climb onto a shuttle and cocoon it. When in humanoid form it was bulky and blobby. Resembling a melting wax figure rather than any kind of solid creature. It could move with lightning speed, snaking out a part of itself (like Odo) to attack, touch or grab. It had no distinguishable face. It radiated rage and sadistic arrogance.
The only Ba'ul we saw also appeared to be dripping with black goo, but as a covering rather than its main structural substance. It rose out of the water, which appeared to be its natural habitat. It's limbs were spindly, almost skeletal, and its long fingers were claw-like. It had sharp spines extruding from its back and other parts of its limbs. Its face was mostly shrouded but it had very distinctive glowing red eyes. Saru called the Ba'ul fragile and, to me, its movements reflected that, almost like it was arthritic in its joints. It relied on the speed and agility of the mobile drones to subdue Saru, rather than risk physical confrontation itself. It came across as cautious, more methodical, almost dispassionate at times.
I really don't think the two of them are related as a species.