What is the optimal time to get from ASEL to ASES certification? I am currently a ASEL pilot, in process of studying for IR, and will then be proceeding to AMEL. I know that if one holds ASEL &amp; IR, and takes AMEL with instrument maneuvers included in the checkride, at the end I will have IR privileges in both single and multi.

What is the most optimal (time &amp; cost) route to get my ASES? Are there any efficiencies to be had by completing seaplane rating after IR and/or multi? Or if I was to complete ASES before IR, once IR is done, I could exercise IR privileges when operating a seaplane (unlikely scenario, just want to understand the rules). From my understanding, I will, because IR applies to the category of airplane. Is my understanding correct?
The instrument airplane PTS has both maneuvers for single and multi engine airplanes. Obviously, when you first get your IR in an ASEL airplane you cannot complete the maneuvers required for multi-engine airplane.  Hence, if you choose not complete those maneuvers when you take your AMEL checkride, you will be limited to VFR.

There are no specific IR maneuvers in a seaplane so once you complete your IR in a ASEL airplane, you will also have IR privileges in a ASES airplane.  

The answer is found in the instrument PTS / ACS book.

If you were to have both ASEL and AMEL ratings and took your initial IR in a AMEL, it would be valid for all aircraft that you are rated in or get rated in later on: ASEL, ASES, AMEL, AMES.