Do Orthodox Christians need their spiritual father to approve any changes in their prayers rules, or is that just a monastic thing?
It's a monastic thing*. Although it's certainly a good idea to consult with others (like your parish priest) who have more experience/training/etc.
*The exception is if you receive a particular prayer/prayer rule/etc during confession. In that case, yes, you need to speak with your confessor before changing it.
