The whole Tommy Westphall Universe thing is somewhat flawed for the very reason that it's in his mind. The kid imagined St. Elsewhere, so what's to say he didn't imagine characters from TV shows he'd seen in the real world interacting with the characters of St. Elsewhere? It's like saying if I had a dream that Homer Simpson was hanging around with Chandler Bing then that means all shows connected to those two shows exist in the same universe.