I've had even unrelated does that lived together peacefully for years, and sisters or mother/daughter combinations that seemed to have been fine until one day they decided to try to kill each other. You can't be sure how any two will do; you just have to watch and see (and be prepared to separate if necessary).
On the matter of hay - yes, of course it's a good thing for a rabbit to have hay available, but that business about "a ball the size of the rabbit" is just silly. Is that compacted, or loose? What are you supposed to do if the rabbit doesn't eat it - put the hay in a blender and force-feed it to them? IMO, a lot of the people giving advice on rabbit sites are young folks with a great deal more enthusiasm than experience (some may even be bunny-owner-wanna-be's, never having owned a rabbit in their lives). They mean well, but with little to go on, they wind up parroting each other.
The old, standard, general rule of thumb is one ounce of pellets per pound of healthy body weight per day, and free choice hay. Now, some rabbits won't eat even that much pelleted feed; others will snarf it down quickly and ignore the hay and spend the rest of the day trying to convince you that they are starving. I say, use your common sense. If the rabbit uses the hay mostly as gym equipment, that's fine, but you don't need to replace every stem that falls through the floor (though you do need to be diligent about removing hay that gets pee'd on). Rabbits' bodies aren't designed to carry fat around, so you need to feed what will keep a rabbit at a healthy weight; that may mean making adjustments that deviate from that "rule of thumb." In the horse world they have a saying, "if you ask four horsemen, you will get five opinions," and that seems to apply to rabbit people, too. Don't sweat the small stuff - if your rabbits are healthy and active, and you aren't doing something way out there (sorry, so-and-so's dad, but I consider feeding a rabbit dog food as "way out there"), you are fine, enjoy your rabbits - that's supposed to be why we have pets in the first place.