Re: episodes that are part of two different TV series
The Simpson's also crossed with The Critic and The Flintstones crossed with The Jetson's. In a way The X-Files also crossed with The Simpson's. Isis crosses with SHAZAM.
It's hard to connect cartoons into all of this given how loose of a contnuity or reality there is. The people on the X-Files aren't four-fingered odd looking yellow people who never age.
Just because it's futuristic doesn't mean it's practical.