Since rabbits can't be spayed as early as cats, usually the puberty hormones kick in some time before they can get spayed/neutered. I know quite a few people on the rabbit forum I'm on who have had to separate their pairs of littermates, get them spayed/neutered, and then rebond them. Some of them never rebond. Some of them have actually caused injuries to each other while fighting before they were separated, and I even know of one very unfortunate death.
I do think getting an adult or two is the best way to go. Some shelters and rescues will have bonded pairs available, or are willing to bond two of their rabbits for you.
As far as rabbits and cats go, it really depends on the rabbits and cats.
My 10 lb foster bunny gets along great with the cats. He's very laid-back, plus he's big enough to be a little scary to them. My dwarf hotot girls (2.5 and 3.5 lbs) do pretty well with the cats, and so does my 5 lb dutch boy. My teeny tiny Polish/Nethie girl though, she's another story. She hates the cats. She's very aggressive and would probably hurt my kitties before they hurt her. I actually know several people with tiny dwarf bunnies that terrorize the cats.
I did have to teach my cats how to behave with the bunnies, but then again they were kittens. I kept the cats on harnesses and leashes until I could trust them.