So technically if the JAR-FCL FI has a SEP(A) class rating on his ticket, and has had the appropriate differences training to fly microlights, he would legally be the equivalent of a microlight instructor
Yes.
A JAR-FCL licence holder with Valid SEP and FI ratings can after appropriate differences training provide training on microlight aircraft.
Those holders of the old UK-PPL fall foul of the requirement to have either an AFI or FI (Microlights) rating before providing such instruction unless the CAA give permission.
For a JAR-FCL PPL with valid SEP to fly microlights then differences training is required.
Regards,
DFC
Regards,