A fox will more or less go where he wants provided he can get an easy and quick way back out. A low fence won't stop him. I am lucky so far, haven't seen any foxes, but perhaps the dogs put them off, I don't know. I only have 4ft and some of the chooks jump that so they wouldn't be safe if there were foxes here. I let them out in the front half acre for a few hours a day. They are inside from dusk as are the ducks, and also if I am to be away for the day.
I would think about a small secure area with a high fence, and a bigger area round it that they can be in when you are around. The electric fence might deter a fox, unless it is low enough to be jumped over..