We are using a smart campaign to add leads onto an engagement program stream. Do I need any transition rules within the stream to pull the leads into the stream?
You need a trigger in a transition rule to pull leads from one stream and add them to another. The idea is that the trigger listens for some lead behaviour to pull the lead form another stream into that stream.
Remember to always think of nurturing with the concepts of Entry and Exit conditions. Each condition needs a smart campaign that is outside of the Transition Rules...these can be a one time or recurring batch or a triggered campaign. But you do need to kickstart it before the Stream's Transition rules take over.