Normally the command that you use is only a debug purpose resulting only the mem in use by newton something like this.
So in the case of multiple Newton worlds
I don't see really the problem in case that you have many world because normally the class have the world in self too,
and if you have multiple world normally you have multiple class instance for idle your world's.
It depending more from the way that you use for implement your newton world class instance's.
Edited:
If you need to use different class with different world, you can all times create one base class with a sort of ID.
You create all other different class based on this class, and you use the pointer base class and you detect the good class type by the id result...
Or maybe i totally not understand you right, if it is the case sorry.