Should this movie have a musical number, starring Mothra's tiny twin spokeswomen?

I agree on the threesome... FULLY!

Unfortunately:

Mothra's Shobijin will not be in the film

Because of the focus of realism in the film series, the magical tiny twin fairies, the Shobijin, will not appear. Not the first time this has happened. Though, there are ways to incorporate something like them however.

Making them human-sized priestesses of a tribe that worships Mothra.
They would fill the same role, being her herald, giving information about Kaiju to the protagonists, and singing her song.

Not having them at all, but including small homages to them.
Like having a pair of twins stare at awe at Mothra, just like in GMK.

Chances are pretty good Zhang Ziyi's role is whatever the equivalent to the Shobijin are going to be. The idea they haven't named her character yet and only played with the idea it connects to Monarch leaves it kind of open. I would not rule out possibilities she as a Monarch employee learns of her family connection to Mothra or Mothra's followers infiltrated Monarch for Mothra's protection. Regardless whether they are actual fairies or some kind of cult in a monastery there will probably be a Shobijin equivalent.

That being said we should also openly admit at least part of this fanbase will be kind of upset if Mothra loses some of her classic splendor in the name of Hollywood realism. Especially when we've got someone like Michael Doughtery as director here who based on his previous work seems like a solid fit.

Let's hope they omit the goofiness that was the tiny twins.