The cooperation was mostly just a wild thought, it really doesn't make sense for to partner up to murder.
I just rewatched the second half of the episode, this time paying attention to detail, and here are my thoughts:
-Togami mentions all of the victims of Genocider Syo are male, which is should concretely reveal Fujisaki's gender. (Kirigiri was also shown at some point to be holding info cards with only female victims, and Fujisaki was absent from them, this could also lend itself to it).
-The extension cord was taken from the library, which puts obvious suspicion onto Fukawa, though to be fair, anybody could have gone in there at some point.
-Fukawa mentions to Togami that she couldn't prevent Genocider Syo from killing this time, which is an altogether strange line. That coupled with her obvious social issues makes my initial thought be that Genocider Syo is some alter ego of hers, because of a mental disorder. Examining that theory, though, I don't really have any evidence, and it seems unlikely that somebody so frail could use the dumbbell effectively to murder somebody.
-Relating to the previous point, In the HorribleSubs/CR subtitle, Fukawa says she won't let Genocider Syo do as she pleases anymore. However, listening to the Japanese, all she says is "Kore ijou wa mo, watashi ga Genocider Syo no suki ni wa sasenai kara." The translation seems accurate enough, but the Japanese doesn't seem to reveal information on the gender of Genocider Syo, so this might be an inadvertent spoiler by the translators. However, it works as confirmation to my guess.
-In the scene where Sakura drops her drink on the mat, you can see the male idol group poster, which is shown to be located in the boys locker room when Naegi investigates. Unless somebody was switching things around, this means she was probably in the boys locker room at some point.
Based on this info, Sakura is still my prime suspect, though obviously there is still a bit of missing information regarding Fukawa.