Mal's case is not an exception.
Initially limbo is basically unconstructed dream space. Let me explain that - Any dream space like the ones shown in the movie are constructed by the architect (e.g. Ariadne). These dream spaces are either filled into the dream by the architect if the architect is one of the people in the shared dream or they are taught to the main dreamer, whose dream is being shared by many. So essentially when the dreamer starts dreaming either he/she or the architect fills up the dream space or rather constructs it. As Dom explains it - in the dream world we create and perceive simultaneously but these processes can be separated and they are for dream robberies and they are done so by taking over the creation part of the dream.
In general when you are dreaming and you die in a dream you wake up. But if you are sedated you cannot wake up, so you are pushed to a further dream state.
This is basically the limbo or the unconstructed dream space or the deepest part of the subconscious. It is unconstructed because you cannot plan for that level in the dream. So the only way to construct a world in the limbo is by building something there while in the limbo or in simple terms go back to natural dreaming process, create and perceive the dream world simultaneously.
Since the dreamers share the dream state if one or more people die while in a shared dreaming experience they will basically be transported to the limbo. If you have already been to that state in the dream before and are returning to it you will find whatever you left there the last time.
Now this is also a level in the dream so this will also be shared. E.g. Dom's limbo and Saito's limbo are connected in the end. So if you die in a shared dream and you friend had already been to the limbo, then you will also find what he left there the last time.
Limbo is essentially where your subconscious is materialised. Dom's subconscious was primarily filled with Mal, his children and his guilt of killing his own wife. Since these are always in his subconscious they are always going to be in the limbo which is simply a reflection and realisation of the subconscious.
Hence the reason Mal exists in the limbo. Ariadne is taken to the limbo by Dom, Ficher is killed in the dream which is why he exists in the limbo, Saito also exists in the limbo for the same reason and the people with Saito in the limbo are Saito's projection of his own subconscious, his army of mind police desperately trying to save him in his predicament of being stuck in the limbo.