NIC stands for Neat Idea Cubes, which was the “original” brand name for the cubes. Now you can find them anywhere and with generic names, but the name stuck–kind of like kleenex and vaseline :p
If your rabbits will spend a significant amount of time in their cage, then they will need to have a large cage, but if you are able to rabbit proof and let them out every day, the cage doesn’t have to be as large. I think the guidelines for a rabbit cage is 2 by 4 for each average-sized rabbit (at the base) and an average sized rabbit I THINK Is considered 6 pounds.
My rabbits’ cage is quite a bit smaller than that (a little over 6 square feet at the bottom) and there is three of them in there, but I let them out to play often and they’re still happy rabbits. The only time I really had a problem was when I was bonding, and I couldn’t let them all out together–so that might be something to think about.
Adults can bond to adults and many of us on this forum have done it! However it’s a big process and you want to make sure you’re ready.