Buddy, Kostubira is listed among the Hermitage principals on this link, with bios. She is a 2000 graduate of the excellent Perm Academy and has been dancing with Hermitage Ballet for the past two years, according to this. Interestingly, other principals of the troupe include Mariinsky principals and soloists, such as Irma Nioradze, Tatyana Tkachenko, Anastasia Kolegova and Sergei Popov. Check this out:
Natalia Potekhina is not on the above list BUT I found a separate page for her (from another private commercial site offering tickets to Hermitage performances). She is a grad of the ballet school in Kazan, Russia:
Now let me search for Valeria Vasilieva...OK, I found her in a news article from Bashkiria, where the Nureyev Festival is held periodically. One 'Valeria Vasilieva' is mentioned as a soloist of Moscow's Kremlin Ballet.
I am venturing a guess that all of these dancers have permanent companies but do lucrative "guest gigs" with Hermitage Ballet, during different seasons and/or on tours. Tkachenko was definitely part of last year's tour through the Southern USA.