What the hell is Slimer, anyway? I don't think he could be the ghost of everyone.. do you suddenly forget how to talk when you die? It's all so very confusing.
In the comics of the Real Ghostbusters, Slimer had his own half-page sketch, but in the one sketch it covered Slimer's past life (he was called king Remils, who was a greedy obese ruler who had died of heart failure.)