A question about whether the classical Stoics subscribed to Plato’s ideal forms was brought up in one of the Facebook groups. 

My answer follows:

No.  There is clear taxonomy of what constitutes “Something” for Stoics.  Bodies and incorporeals are Something.  Everything else is not-Something.

They use the common definition of a body as that which can act or be acted upon, and has extension.  Matter disposed in a certain way is also a kind of body (hand vs fist). The forms fail that test.  We have never encountered​ a universal red-body, for instance.

So if the forms are Something for a Stoic, they must be incorporeals.  Let’s look at that.

Incorporeals are limited, void, time, room, and lekta (propositional content).  In modern terms, incorporeals supervene on bodies.  They can be described physically, but rely on bodies somewhere for their subsistence.

Take the idea of traffic.  We can describe its flow, volume, path, speed, etc.  But absent the cars, it does not exist.  Traffic supervenes on the bodies of the cars.  It subsists, but does exist (only bodies do that).

The forms do not meet this criteria, either.  They are an idea of, say, “red” of which every red thing contains a part.  The forms come first, they do not subsist on a body.

So, the forms are neither bodies nor incorporeals. Therfore they must be “not-Something” according to the Stoics.

For more on this topic, see (now) Dr. De Harven’s dissertation: https://escholarship.org/uc/item/3wg7m1w0