They could only have 4 of the 7 castaways because Tina Louise would never play Ginger again. Jim Backus was at this point too sick to play Thurston Howell III, and it would have made no sense to have Natalie Schafer play Mrs. Howell without Mr. Howell. So Ginger and The Howells are only mentioned.
While talking with Lynn Tanner (Andrea Elson) about a deceased pet turtle, ALF says, "Sic transit gloria mundi" which is a Latin phrase that means "Thus passes the glory of the world".
When "The Tanner Family" comes on, Willie, Kate, and Lynn are credited as "father", "mother", and "sister", respectively, but Brian is credited as "himself".
The TV show the castaways were watching was called "The Tanner Family", which was basically "ALF" without Alf.