We do have:
10.44a. [Bailey] "For on the one hand the nature of the void which separates each atom by itself brings this about, as it is not able to afford resistance, and on the other hand the hardness (stereótēs) which belongs to the atoms makes them recoil after collision to as great a distance as the interlacing permits separation after the collision."
stereótēs is "the condition from making stiff" and means firmness or hardness.
"Totally changeable and soft matter" is for the other schools -- it is the exit door out of science and into religion.
"The Stoics together with Heraclitus say that matter is wholly and completely changeable and alterable and mutable and fluid." (Aetius 1.9.2)