I doubt we will see Leonor in a tiara soon, the emphasis will be on her education and I don't see her participating in State Visits straight away.
Regarding the tiaras used by the SRF, Leonor will certainly not wear the big guns until she either Queen (FdL) or really settled in her position:
Loop, Fleur de Lys, Pearl.
In the beginning, Leonor and Sofia will probably share:
Floral, Shell, Prussian (owned by QS), Princess (owned by QL).
Do the Niarchos rubies count as tiara?
But I do hope that QS will gift both the rubies and the Prussian Tiara to the Crown and not give any substantial jewellery in that sense to her daughters.
VF could always borrow something from her uncle in case she fancies a tiara at her wedding.