From what I have observed, hackney's were bred for big movement..
If I had any advice to give it would be study their personality, as some can be a bit tricky to deal with.
Ive had 2 hackey mares on my property for about 2-3 months, and they kept me on my toes.
They were very aggressive towards any of my smaller miniature horses,- even when put in a large grassy pasture so I had to seperate them.
These two mares I had were rather obstanate when working with them, and finnally went to a professional trainer in a trade deal.
Ive also owned a miniature mare that was part hackney, and she had very nice movement, but she was just a tad goofy in the head.
Her behavior was a bit unpredictable, so I was the only one that regularly handled her in my family.
I bred her to my calm and wonderful stallion who also happened to compliment her conformation, and her babies were amazingly good to work with, and not goofy at all. ( one is in my avatar)
So the goofiness can be bred out with the right cross.