It's a good idea to keep your hens in their run for the first few days, as they will be so stunned!
When they first free range, you will have to put them into their house each day, just before it gets dark. They will soon get the hang of it!
It sounds like you have fairly good security, rats are much more of a threat if there is eggs or food lying around. Rats can kill chickens, but i am pretty sure it is rare, they often kill chicks.
Foxes can slide bolts but a garden which is hard to access should not be such a problem. What can help me is seeing it from the fox's point of view, i.e. 'Could i dig in here? Are these hens locked safely in?'
I hope some of that is helpful, and good luck with your hens, I am sure they'll be very happy!
