Hello Claire!
The transition rules will apply to your leads even if the stream they move into hasn't currently been made live.
The lead will remain in the new stream until the cadence is set up and it is activated.
So for example:
Stream A is active
Stream B is not active
Your lead spends with you, and your transition rule pulls a lead from A to B.
Your lead remains in B, but nothing happens until you activate B.
Hope this helps!
Louiza