What is the optimal time to get from ASEL to ASES certification?
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.