I have used many different horses and ponies for this sort of work. Majority of animals will adjust to the disability of the riders and behave appropriately.
I will state that unless a rider is greatly lacking in balance I will only have one walker alongside them and their job is not to hold onto a belt the rider is wearing but to just keep a hand on their leg if necessary.
I went to a national course for RDA (Riding for the Disabled Association) work. One year it was based on the suitability of ponies/horses for the work.
It started with about twenty riders coming into the arena riding horses that they thought might be suitable for the work. They all rode around in single file to start and then were asked to trot and then canter, then to work as individuals.
We, as an audience were asked to select which animals we thought were unsuitable. We had to write their numbers down and submit them after watching these for about an hour.
During this time the instructor did all sorts of things like kick a ball towards various animals, walk around twirling a golf umbrella, making sudden movements towards an animal and then setting up four bleepers (used for blind riders) in the corners of the school.
I crossed off three animals in the first 10 minutes. One was only three years old and very green although of a very steady temperament. Another was an older pony that was obviously suffering from COPD as it kept coughing and in doing so would yank its head down. It also stumbled a lot. My third was an Arab that was spooky and kicked out at several of the other horses. The final horse was one that was so lazy it would stop when only walking. It also had a mean eye and something in me just did not like it.
There was a cracking pony that behaved impeccably until the bleepers went on and then it was something terrible. Spooking away from the corners and getting very upset over the noises.
The rest of the animals were fine, yes, they had faults but nothing that could be considered unsuitable in my eyes.
The instructor had never seen any of these animals before and a prize was to be awarded to the person who agreed with his!
As we had a break we were all chatting over which we had all crossed off. Majority had discounted seven or eight. I was the lowest with only four on my list.
Next morning I was called into the arena and explain my selection. It was the same as his. All the ponies had been called into the arena and as I explained why so each left.
The big question was why not throw out the pony frightened of the bleepers. My answer was that it could be trained to accept the noise. Much to my surprise the pony was called in and all four bleepers set around him, when turned on he never moved except when asked. He took o notice of them! The instructor had set them up in the stables and left them on all night.
The following day the instructor brought in a group of his riders. None of these animals had ever seen a wheelchair or a person walking with sticks. A couple looked but then relaxed. The kids all mounted (one wheelchair bound lad did so on his own by holding onto the crest of the pony's neck and hauling himself up!)
These children all rode without a leader or side walker after about 10 minutes. The animals all behaved as if they had been doing it all their lives. They adjusted to different aids yet went willingly into faster or slower paces.
What I did also learn was that these people with various degree of disability were all capable of riding without any help.