If I have a campaign with wait steps and email sends, will the email be skipped if on that day we've reached our communication limit? Or will it be sent the next day?
The email will be skipped if the lead comes out of the wait step at a moment when he has already reached the communication limits for the day or for the week.
But you can workaround the thing with the following steps:
Unfortunately, there isn't a way to identify such leads easily. There is a Skipped, Communications Limit log entry, although I'm not sure this appears outside of Engagements.
You can always set a specific flow to Ignore Comms Limits.
Engagements will send the Cast that was skipped.