Do you think certain talents are contractually prevented from collabing with males? I have no proof but for as much of a unityfag Sora is, she's never so much as mentioned STARS. Same with Gura. I imagine this is because they are so integral to the brand that Cover doesn't want to risk an otaku uprising.
On the other hand, Kiara has mentioned STARS but only to say she wouldn't collab with them. That makes me think she COULD but chooses not too.