There is no such limit in shipping! Anyone can always be paired with anyone. It's just that the author has given a certain pairing more screentime and they became popular.
[No ship war] but in Love Live, the reason NikoMaki and KotoUmi are so popular is because of the studio giving them more interaction. Though, HonoMaki, NozoNiko and even UmiPana can turn popular as well if they gave these pairings more screentime. [End]
On side note, we didn't have the chance to really learn more about Yuyu and Nyunyu. I am sure these two would turn into nice yuri characters~