MongoDB Indexing - How many fields should I index?
It seems that your collection is quite small (5MB?). I would create one index for each query and check execution plans.
The limit of creating too many indexes are :


If your indexes are too big and don't fit in RAM (I suppose you have
several GB of RAM ?)
It increases I/O during documents insert/update because each time you write a document, indexes are also updated.


Also, indexes are necessary when sort operation cannot fit in memory. There is a limit of 32MB, so here again you are below this limit.

Please refer to indexing strategies for more details.