Memory management scenario with MongoDB
Mongodb uses mmap for memory management, relies 100% on the OS memory management. This means whatever memory you have left it has a really high possibility will be eaten up by mongo cause the entire datafile structure is mapped to virtual address space (It is not certainly a problem while the paging between physical and virtual memory is not that dumb to break your system down). 

While Linux will page out some parts of the physical memory, so not the whole virtual segment have to be in physical memory just the recently accessed parts (check this), depend on the usage pattern mongo can run without problem.  If the OS will have to extensively page things mongodb will be slow.  You can restrict the usage of the memory. I would recommend to use cgroups. For your probable workload memory usage you can check also this documentation to have some idea.