Why do some IAPs lack an Intermediate Fix and/or include other fixes between Final Approach Fix (FAF) and Intermediate Fix (IF)?
The rules are quite lengthy -- a 509 page PDF.  They're all spelled out in excruciating detail in the FAA Order 8260.3D, United States Standard for Terminal Instrument Procedures commonly referred to as the TERPS.  

It's available for download here.   

I would suggest researching your questions there, and returning here if you have a specific question regarding the application of the TERPS.