This is the research I've done to figure out which character from the anime in Sona Sitri's Peerage is who. I'll be doing this from the one I am most certain of to the one I am least certain of.
Image Reference of the Sitri Group:
Momo Hanakai (Bishop)
First up is Momo Hanakai.
"Hanakai-san was the girl who was carrying the pack with my blood inside at the Rating Game against Sitri-group."Image
Now compare that image with the girl third from the right in "Sona's Peerage (Gym)" image. As you can see even though their hair color is diferent (since the image from the Light Novel doesn't have color that would be obvious), their hair style is the same.
Tsubasa Yura (Rook)
Next is Yura.
Volume 5, Life 4 "Waltz"
"However, they encounter Sona's Queen, student council vice-president Shinra Tsubaki who is known for sword skills. At her side is a tall girl, Yura the Rook, and a slim girl, Meguri the Knight, carrying a Japanese katana."
Now we know that Yura is tall when conpared with Tsubaki Shinra, now if you look at "Sona's Peerage (Gym)" image you can see the girl third from the left is a bit taller than Tsubaki Shinra while the other girls are either the same hight of shorter than Tsubaki Shinra.
If you followed the link for bishounen you will know it referse to a "pretty boy" which would mean Yura has a male like face, now if you watch episode 7 of the anime at Anime Avenue here go to 'souce 2' at 10:17 you will see her face is the closest to that of a male's face (in my opinion anyways).
Tomoe Meguri (Knight)
Now time for the tricky ones, the twin tails (or pigtails). Stating with Meguri.
Anime, Episode 7 Source 2
According to this Wiki, in Sona's Peerage only two of her servants besides Tsubaki Shinra and Genshirou Saji has voice actors. Those two are Momo Hanakai and Meguri, if you watch episode 7 here at Anime Avenue (remember to click on 'source 2' for the video I used) you will see that Sona's servants where the referees for both the tennis match and the dodgeball game, both being different people. If you go to 10:21 you will see the one I belive to be Momo Hanakai being the referee of the dodgeball game, leaving the one being the referee of the tennis match (07:14) being Meguri.
Now before you start asking "what about the girl who got hit by Rias at 10:45" if the one I belive is Yura at 10:17 doesn't get a voice actor for screeming (or something simular to it) then nor does the girl at 10:45.
Ruruko Nimura (Pawn)
"There is a girl on Saji's back who Ise recognizes as one year below him (Koneko's year)." <-->"Meanwhile, Koneko has started fighting with the girl, Nimura. The girl is competent enough to block Koneko's punches, but begins to lose her balance. With a fist covered by a white aura, Koneko's punch strikes Nimura in the chest, who falls over." Image
My only reasoning for this one is since she is the only 1st year in Sona's Peerage she is most likely the shortest (after all Saji carried her on his back). Now if you look at "Sona's Peerage (Gym)" the girl on the far right is the shortest.
"The one with pig tails, the “Bishop” Kusaka-san congratulates me by saying “Congratulations for your promotion recommendation!” as soon as she see’s me."
Other than having pigtails (gives a vague description) I couldn't find anything else for her but since she is the only one left perhaps their is no need for futher reserch. In "Sona's Peerage (School)" image the one second from the right is the only one left so I think she is Kusaka.
"Sona's Peerage (School)" image from left to right:
Ruruko Nimura, Momo Hanakai, Genshiro Saji, Yura, Sona Sitri, Tsubaki Shinra, Kusaka, Meguri
So if you have anything to say about my research please leave a comment.