I’m a bit confused. Are you still planning on putting a permanent fence inside the outside fence so the chickens have permanent access to the entire area around the garden with another moveable fence to section the garden off into a portion for them and a portion for you to grow things, or are you talking about a permanent outside fence with a moveable fence inside that to section the garden off for them and you? That’s abandoning the moat idea. The big advantage to the moat is that the chickens may intercept some of the pests that attack your garden.
If you abandon the moat idea, you will probably want to put your coop in the middle of the garden so you can give them access to whichever section you want. If you stick with the moat, you can still put in a permanent coop anywhere that attaches to your moat.
With the moat, you need to be able to get inside. Chickens don’t necessarily kill everything in the run. They won’t eat certain plants so you’ll need to occasionally go in there to weed eat them or somehow clean them out. If you are doing that, I’d make the inside fence high enough so you could cover the moat portion and walk in there, maybe with bird netting. That won’t keep anything out, just keep chickens in.
How high does the fence need to be to keep them out of the rest of your garden? A whole lot of that has to do with the motivation of your chickens. I use electric netting 48” high and don’t have a problem keeping them in there. The area is big enough that it is always green. I also avoid narrow areas or sharp corners. When chickens get caught in tight spaces and have a pecking order dispute, the loser will often go vertical to get away from the winner. The adult chickens have no trouble at all of clearing the 48” high fence if they want to, but they don’t. Your moat is a very narrow space. I’d want it covered to keep the chickens in.
Chickens like to perch. If the top of your fence looks like a good place for them to perch, they will probably hop up there just for fun. Once up there who knows which side they will decide to hop down on. Sol whatever you do, do not make a rail on top of the fence where they can perch. Just have it wire.
For the fence that is cutting your garden into sections, I’d probably go with five feet high. Can they fly over that? Yes, if they want to. But I think you will find that they will not want to if they have a reasonable big open area. I could be wrong.
If you put up a permanent fence, you will have grass and weeds growing up in that. It’s a maintenance problem and it is a source for grass especially to spread into your garden. Permanent fence posts will do the same but they are a whole lot easier to keep the grass and weeds down than a fence.
I don’t know how big your area is. I’d consider using T-posts as fence posts. End posts or corner posts will need to be more solid than T-posts but T-posts are great as line posts for this application. These are not that hard to install or to dig back out in most soils. I just use a mattock to dig around them some and wiggle them out. Get that tool so you can drive them in. Get 5 foot high fencing that can be attached by zip ties maybe for easy installation and removal. You can use T-post connectors too if you wish. You might need to install an apron maybe 12” wide around the bottom to keep the chickens from going under the fence. It depends on how well you are able to put the fencing close to the ground. It may not be a problem. To make an apron, take a 12” wide piece of fencing, lay that flat on the ground on the side the chickens are on, and attach it to the bottom of your fence. This same idea is often used outside a run using 18” fencing to keep digging predators out.
There is a lady over on the gardening forum (a sister site to this and run by the same people but using a different format) that has a coop surrounded by different sections where she grows stuff in some, lets some go fallow and rotates chickens and compost in them. You might go over there and post to see if she is active. I can’t remember who it is.
http://www.theeasygarden.com/