Doubts on Setting up Master-Master replication (Percona)
Set up slave

You can set it up from the nightly backups. That's actually a common practice to provision slaves from backups to also have them tested. You can go back as far as you have binlogs stored on the master (expire_log_days variable).

The impact of having a slave starting from older backup is that it has to fetch the logs from the master but that shouldn't be a heavy operation. Unless your master is already on the edge you shouldn't really notice it. The other impact is that it will take longer for the server to catchup which is of course expected.

Switchover

Switchover can have some impact if you use auto-increment IDs and your passive master is not fully caught up with replication. Best is to make the old active read-only for a second or two while checking the passive for master position. Once there is no change you can move change it to be the active and make it writeable. 

Depending how you point to the active master (DNS switch for example) it might take some time during which time both masters will accept writes and the old active being read-only will report errors.

I hope this helped.