You have gotten great advice here. Treat them like any other horse. Knowing their mental limits is a must. And having good timing of taking off pressure is critical.
I know a mare who was seriously abused for a length of time, while tied up in stocks. Horses are "flight" animals, only when necessary will they "fight." This mare learned that she couldn't run away from danger, so humans essentially taught her to "fight." The woman who rescued her thought it would be best to gain this mares trust by being her "friend." Don't get me wrong, this woman is great with horses and very knowledgable. But this is a mistake a lot of people seem to make with abused horses. The mare is still aggressive at times, but some of that is because her new owner wasn't sure of when to punish an abused horse, for fear of losing the horses trust. So now, she will still bite and kick when you saddle her or do things she doesn't like. But she no longer tries to run people down and attack them. Treat these horses like any other horse, they don't need any special treatment. Just good horsemanship.