Frame challenge: this isn’t actually important.
As I noted in my answer to your other question about initiative, this is not a game balance issue. The initiative “rules” about rolling once for groups of identical monsters are there to make your life easier – use them if you want, don’t if you don’t. So it doesn’t matter what identical means here. Just use the method that works for you and your table.
“Identical” means “uses the same statblock”.
The basic English definition of identical is:
Adjective. similar in every detail; exactly alike.
A dragon and a kobold are in no sense identical. Being allies does not make two creatures identical. Being exactly alike in every detail makes two creatures identical, and the details of a creature, at least for the purposes of running combat, are found in the statblock.
It is important to note, this is how I understand and interpret the word. There is no canonical rules-as-written definition.