Can you help?

This is about using commas in sentences.

1) I enjoyed eating fresh, crisp salad.

2) I enjoyed eating fresh tossed salad.

The book says comma is used when the sentence sounds natural if 'and' is used between the two adjectives.

And to make it clear there were two examples which have been given above.

My question are: 1) Does this sentence- An alligator has thick scaly skin- need comma between thick and scaly?

2) How else can I explain this? I have this question because sometimes I am unable to sense whether the sentence sounds natural with or without 'and' in between the two adjectives.
I would put a comma between thick and scaly (thick skin, scaly skin). Sometimes with short, highly collocated adjectives, it is omitted: She lives in a big old house in Massachusetts.
Thank you, Mister Micawber.