This is a mostly reasonable assumption.
Were these Elves' souls in Mandos?
I talk extensively about the fate of Elvish souls after death in my answer here, and none of it indicates that a feä could only be admitted to Mandos after the Elves collectively reached Aman. In fact, one passage implies exactly the opposite (bold is my emphasis, italic is Tolkien's):
[I]n the days when the minds of the Eldalië were young, and not yet fully awake death among them seemed to differ little from the death of Men.
It was in Aman that they learned of Manwë that each fëa was imperishable within the life of Arda, and that its fate was to inhabit Arda to its end.
History of Middle-earth X Morgoth's Ring Part 2 "The Second Phase" Chapter 3: "Laws and Customs Among the Eldar" Of Death and the Severance of Fëa and Hröndo [> Hröa]
So yes, there is every reason to believe that the souls of those Elves who predate the Great Journey were sent to Mandos1.
Could they have been reincarnated before the others arrived?
I'm inclined to suggest "probably not". I talk much more about this in my previously-linked answer, but there are two problems I see:
In some writings, Elvish reincarnation has to be done either through childbearing or into the original body, in the rare circumstances where the original body is basically perfectly preserved.
Elves who died before the Great Journey wouldn't have an opportunity to be "re-born" (in Aman) until the rest of the Elves arrived; they may have been re-born into the journeying Elves, but it's not clear if they would have been recognized by their kin.
And, since they died in Middle-earth, presumably of violent accidents, it is doubtful any of them would have been able to return to their own bodies.
The passage that mentions being re-born into newly-constructed bodies only occurs after the death of Míriel:
Manwë spoke to Eru, saying: 'Behold! an evil appears in Arda that we did not look for: the First-born Children, whom Thou madest immortal, suffer now severance of spirit and body. Many of the feär of the Elves in Middle-earth are now houseless; and even in Aman there is one. The houseless we summon to Aman, to keep them from the Darkness, and all who hear our voice abide here in waiting. What further is to be done? Is there no means by which their lives may be renewed, to follow the courses which Thou hast designed? And what of the bereaved who mourn those that have gone?'
Eru answered: 'Let the houseless be re-housed!'
Manwë asked: 'How shall this be done?'
Eru answered: 'Let the body that was destroyed be re-made. Or let the naked feä be re-born as a child.'
History of Middle-earth X Morgoth's Ring Part 4: "Athrabeth Finrod ah Andreth" Appendix "'The Converse of Manwë and Eru' and later conceptions of Elvish reincarnation
If we take this passage as it stands, it implies that the practice of creating new hröa out of whole cloth post-dates the arrival of the Vanyar and Noldor in Aman; so their fallen kin couldn't have been resurrected and waiting for them in Valinor.
These are, however, getting into some of Tolkien's later scribbles, which sometimes get a bit messy and self-contradictory. At this stage of the narrative, it's not entirely clear what he intended to be final or not, so your mileage may vary.
1 Assuming they answered the summons; as I mention in this answer, it is possible for them to refuse; but it seems that most Elves of the First Age accepted the summons