As EQG revealed, interdimensional travel is not impossible for Equestria, so I decided that crossovers can count as long as the characters from both involved series are in-character. Although it's primarily about the MLP characters, it's also important that the characters in whatever series MLP is being crossed over with in the fic are also in character. In other words, if the ponies meet a character who's pure evil in his/her own series, don't make him/her a good guy; keep him/her evil like his/her canon.