Hi!
So the MassTraffic is a very complicated setup of rules and managers for ONLY the vehicles so they can merge, change lanes, avoid, react to traffic signals, and manage the parked cars. It is not an official plugin for the engine or supported outside of the City Sample project.
MassCrowd is a collection of common behaviors we had for simulating the pedestrians in the CitySample, and it is an official engine plugin that is experimental. It builds on top of our other Mass plugins for Gameplay and AI. It contains some of the StateTree tasks used in this tutorial such as the Find ZoneGraph Wander Location task and some navigation code directly made for the pedestrians. The plugin also has support for waiting for lane closures e.g. waiting for the crosswalks to be open at the traffic signals.
For your desire to have multiple humans appearing randomly, I would think to use the Crowd instead of the traffic plugin. You can have multiple entity configs created that have different classes for the visualization/representation and add them to the MassSpawner with the ratios you want them to be used. Alternately, you can use randomization either inside the BP for changing the mesh. We use a setup similar to this in CitySample where one entity config and BP class is used, but there are several different MetaHuman meshes used for the crowd.