I'm with you that one guy needs to step up and become the clear starter out of the group but I think that any of them can be a force because they all have strentghs that they could use to explot a defense.
I like to pretend I'm a coach sometimes lol and I would like to see Wallace come in and be used in a role similar to Reggie Bush at USC. Have him play in the slot, at HB, and returning kicks. Then I would use abdule Leveir and Chester Johnson as my primary HB's and Dwight Lindon on short yardage. Then Harrison and Rubin can both be solid 3rd down backs because of their speed but not every down backs because of their size.
Our depth at HB should be used to our advantage because when we are pounding the ball in the 4th quarter and all of a sudden we throw in a fresh back like say Celeb Rubin and all of a sudden you break a 20 yard run because a tired defense won't be able to keep up with him.
I had mentioned Jason Cherry's name because we don't know whether he will be a WR or a HB.