Rob A. wrote:
laihana oleva kissa
laihina olevat kissat....I think these are grammatically correct....and that these would be considered two adjectives followed by a noun...and in this instance I think they would be described exactly the same way in English.
But I'm not really sure when and how you would use them..... and why you would just say ...
laiha kissa/laihat kissat....Maybe a native speaker could give a few sample setences ...

Not a native here, but some observations:
As Jukka points out, there's something odd about doing this with
laiha (though not with
paksu 
). The
-na form is about a
state, I think, and most often a mutable state.
And more than that, as with so much about cases, it is about
context. You happened to use an ambiguous verb here, Rob.
Olla can often go either way, nominative or essive. Two adjectives frequently used in essive are
sairas and
terve.
Olla sairas and
olla sairaana are both possible, as with
olla terve and
olla terveenä. It's tempting to say that
-na is used for more temporary illness or health (and nominative for more pervasive or character-related states), but there is only a germ of truth in that. It's not sufficiently true to be useful in choosing a form for a sentence. What is more useful for that is just knowing idiomatic usages.
Here are two different contexts in which I would almost exclusively use one form or the other, but Jukka or someone will have to say if there's foundation for that:
Hän on vakavasti sairas.
Hän on ollut nyt kolme viikkoa sairaana.
To me it would feel very awkward to say either one of those with the other form (more awkward with the 2nd than the 1st). If this is a valid distinction, and it feels to me like at least a > 50% valid one, I still can't say why that is so, can't describe what it is about the respective contexts that so much attracts one form or the other.
But as I said,
olla is a squishy verb. Consider instead
pysyä or
säilyä/säilyttää. I know there must be others like these, but at the moment these are the only two that come to mind. They both demand
-na for a state of being. Contrast these to a verb like
pitää which is often used with essive (
Minä pidin häntä hulluna;
minä pidin sitä selvänä, että...) but has many other uses as well.
I mean there are some exceptions like
Pysyä poissa or
pysyä aikataulussa because the words are not adjectives. (And in a way, the function of -na is to make an adjective more like -ssa, which I suppose is reflected in the respective terms essive and inessive.) But with
terve or
koskematon or any other adjectives, you
have no choice but
pysyi terveenä or
säilyi koskemattomana. And somehow that makes perfect sense, though again it's difficult to say exactly why. Much as in English we might say "It's a pristine lake" or "Aggressive conservation efforts have
kept Lake X
in a state of pristine clarity and beauty," there is a distinction between the essential quality and a mutable or transient state of possessing the quality. Of course we could almost as well say "kept Lake X pristine" or "kept Lake X clear and beautiful," but you can see that there is an impulse in our language as well, though a weaker impulse, to somehow make this distinction. And it's a distinction one makes for pristine but not for beautiful/beauty, because beauty is usually treated as more inherent than pristineness or
koskemattomuus. It's an interesting exercise to play around with endings for the latter sentence and see which ones feel OK with "in a state of" and which ones don't. It's a little awkward because it has to go "in a state of
adjective" and then you have to choose one or more objects for the adjective. But you end up seeing that not all adjectives are created equal.