The fish had been cut into well-formed bite-sized rectangles
Like in this example, is it necessary to add a comma between well-formed and bite-sized?
Why or why not?
I would ignore the fact that both of your modifiers are compound adjectives and treat them as if single words.
There is something called the Royal Order of Adjectives. It has no logical basic: it is just the natural order people tend to prefer with multiple adjectives. There is a priority of categories. Two modifiers in the same category need a comma separating them, but no commas are needed when your adjectives are in the order which just sounds better.
What does this sound like?
bite-sized well-formed rectangles
That sounds equally natural to my ear. I would therefore choose to use a comma. However, others might disagree, think the original version sounds better, and choose to use no comma. I would not say they were wrong; this is not an exact science.
EDIT TO ADD:
Linguists who have classified the order of groups place "opinion" ahead of "size and shape". See this link.
That suggests it is more natural place to 'well-formed' before 'bite-sized' - and therefore to not use a comma to separate them.
I think I was too hasty in my choice about them sounding equally good, and would now recommend that.
Imagine you are saying this sentence out loud. How long of a pause would you put between "well-formed" and "bite-sized"?
Much of the time, this is what the comma is for -- to indicate the pauses that occur in verbal communication. For this reason commas are almost always optional. Where they help the reader parse the sentence, it's nice to use them, but it's rarely required.
Here's an example of what I mean:
As they rushed around setting up food for the hungry children, a fat mottled larcenous seagull paced a few feet away, suddenly darting in to carry off an unguarded sandwich. "Oh you rotten thief!" cried Lucy, hopelessly running after it.
I suspect most writers would use commas, "A fat, mottled, larcenous seagull," but some writers may choose not to. The lack of commas can make the sentence feel more hurried, or more chaotic. In contrast, using commas can feel calmer, and more organized.
In this case your two compound adjectives are clearly separated, so I personally feel no comma is necessary -- however, using the comma is not wrong.