Long Answer: There are minor limits. Any new character must be relatable to any current character, I mean by story and series. If your new character is not relatable to any current character, then it must be or grow into a group who are relatable to eachother. God gave you a brain, so any character that you see as janky or weird is not reccomended, but limits end before this point is hit.
Short Answer: Little limits. Characters must know each other. Groups or pairs who are unknown by any current character are allowed.