Handle and Head compatibility

At the risk of being disrespectful. Are the fit and form of handle and heads standardized in their hole and threads such that I can purchase just the baseplate and cap to use handles from other brands?

M5x8 and 10-32 are the standards.  Slightly off one another.  Long term use you should honor the standard.   Some razors like Tatara have unique portions of the upper handle that will only fit well with their razor heads.  But Tatara handles are interchangeable on the Tatara non-adjustable razors.  As long as you match the threads there are numerous handles that will work with a lot of different razors.  Some forms have threads on which razors have M5x8 and 10-32 threads.

An M5X8 threaded handle will generally fit on a 10-32 threaded head, but not the other way around.  
Fatip Razors uses a different threading standard, and are not interchangeable with other razors.  
Also Aylsworth Razors uses Higbee Threading, and their handles will not fit any other razors but their own.