Help with moon cactus!

[Blooming]

Hi! I'm houseplant enthusiast with many nice ones in my small apartment. On a recent trip to Home Depot I saw a little pot with cute cacti and decided to buy it. This is my first time taking care of a cactus. I was so distracted with the other plants I bought I didn't realize the black gravel top was glued in! I wouldn't have bought it if I had seen it. Last night I carefully removed it and repotted the four cacti. I hope they're ok!

I'm attaching pictures of the extra moon cactus top I found with the root of another cactus, and the whole thing together. What should I do? Can I graft it to the other one?

I had no idea what a moon cactus was and I'm glad this experience led me to discover cacti. I hope to take care of them very well and add to my collection.