Also, several characters from St. Elsewhere once appeared on Cheers, and since St. Elsewhere is just a figment of an autistic child's imagination, then Cheers, Frasier, The Tortellis, and Wings are also a figment of the child's imagination.
More likely, the
St Elsewhere characters are real people that appeared on
Cheers and also encountered the autistic boy at some point, thus leading to the autistic boy integrating them into his own fantasy life.
Exactly. Just because Tommy Westphall dreamed something (like
St. Elsewhere) doesn't mean it didn't really happen. Obviously some differences would have to occur, given who Tommy's father and grandfather turn out to be, but the bulk of things can still be the same.
Besides, some crossovers which appear to be the case obviously cannot really be such. For instance, John Munch appeared on
The X-Files, which is itself fictional within the shared Homicide/L&O universe (in an early H:LOTS ep, Munch says something about "on Friday nights, most people are home watching X-Files").