And I like also to add, we need to distinguish the concilia Plebis of the 494, it means the council only for the plebs, and the meeting of the tribunes, that was a assembly were plebs and patritian were reunited, but this is from late age.

The concilia Plebis are born like a one side assembly (for the plebs only) the plebi scita commit only the plebs. This uintil the Ortensia law were the plebi scita commited everybody (including the Patritian).

About the senate, at the beginning there were also plebeian consul (nothing prohibit that) and there were also plebeian consul in the repubblic, but they were rapidly excluded from the office. Later, when the law change the situation changed.