Your predator assessment will determine what type of fencing you need.
Chicken wire is great at containing chickens. Run it around the perimeter behind your hot wire and your birds will stay inside the area. But, it won't do a thing to keep predators out.
Electric poultry netting is supposed to be great at keeping most predators out. If aerial predators are a concern, you will need hiding places for your birds. The horse's shelter is fine, but it may be too far away. Shrubs and bushes are good, but not always feasible in a paddock. Some folks use something as simple as a pallet on some cement blocks, just raised up high enough for the birds to get under and take shelter. You can be pretty creative to find ways for them to have cover from birds of prey.
Chickens do well with low voltage electric. Just be aware, the electrified poultry netting can have large enough squares that smaller chicks can go right through it, so littles will need to be otherwise contained until they're large enough to get zapped as they try to go through.
I think you'll enjoy having chicken with your horse. They scratch through the manure and disperse it so there's less clean up, and less flies as they eat the larvae. But, depending on how tolerant your horse is, the birds might start using her as a roost during the day, and you may wind up with chicken poop on her back
Edited by donrae - 3/17/16 at 8:58am