I would never tell Izzy to hit back, I dont believe in violence of any kind, so blurring the edges by saying its ok to hit if you are hit seems wrong somehow - and at a young age it could be quite confusing for them.
I HOPE to have a relationship with Izzy where she feels she can tell me anything - I know thats often the problem when kids feel bad somehow for being bullied and so they cant tell anyone and it carries on for years :-(
I think rules for children should be as simple as possible, so for me its 'NO hitting', not 'no hitting, but if you get hit then its ok to hit back'
Its the same for adult rules - no drink driving means NO drink when driving, not 'one will be ok' - that way its easy not to get it wrong!
I don't really know much about the whole physical bullying thing as there was never that kind on school, there was tormenting bullying - which when ignored tended to move on to more 'interesting' subjects who bit back!