Everyone have this idea that Rocky and Simon would have a rough start, and I understand why. However, I feel that Rocky would totally freak out if he gets to meet Colt (in the twins AU), and I think it would be very funny if they just hate each other because they are competing for Grace attention and love.
Think in the possibilities (with some help of the magic of fanfiction)
Rocky mixing Colt with Grace because they have the same or pretty similar heartbeat, or maybe they look the same for Rocky echolocation with very little variation. Colt, knowing that this frustrate Rocky uses to his total advantage to mess up with him. Sometimes Grace would help him, because it is funny.
Both of them calling one of his kids Grace in honor of Ryland, so know they are making baby Grace and pebble Grace compete against each other.
Rocky presuming that he can talk about science with Grace, and that they have a strong bond thanks to their whole odyssey in space; Colt been super clingy with Grace because he can touch him and they share a "twin link".
Rocky not believing about the "twin connection" because there is no conclusive evidence about it, while Colt is like "of course we have a twin connection, we can read each other mind, if he sneeze, I sneeze." and Rocky been like "Colt a liar, statement. Grace told Rocky that humans not read minds", Colt answering without blinking: "Normal humans don't, but twins do."
Grace having to calm them down and promise them that he loves both of them.
Rocky showing all his abilities as a engineer, and a alien that is super resistent; Colt showing all his tricks as a stunt (Both are impress by the other, but they would never admit it)
Colt been introduce to Grace's class. The pebbles been mesmerized to the idea that now there are two Grace, and having a bunch of questions that Grace tries to answer the best he can, while Colt is just playing around.
Extra + Simon:
Simon not knowing what twins are (maybe the gen was lost through time and the rupture, or just he did not know, because "cult").
Simon been introduce to Grace first, and then Colt, but not knowing that they are different people, and thinking that Grace just has gone a little bit mad thanks to his time in space, but he is no one to be judgemental.
Simon and Rocky, who dislike each other, doing an alliance to figure out what twins are.
Simon starting to think that depending the clothes and glasses that Grace is wearing he would change his personality. ( normal glasses are nerdy Grace, sun glasses are risky Grace).
Simon not knowing if he is interacting with social awkward Grace or suicidal Grace (Extra points if Grace and Colt are both social awkward and suicidal just in different flavors).
