Heinlein addressed this to some extent in the book. He outlined three means of communication, one of which is a version of another. The slugs can communicate from host to host, either directly or long-distance. They could communicate from ship to ship or ship to base, which would appear to also be host-to-host communication. And they could communicate via slug-to-slug conference.
In chapter eight, he writes:
One of the disadvantages we worked under in serving our masters-or perhaps I should say one of the disadvantages our masters worked under-was the difficulty of long-distance communication. It was limited to what human hosts could say in human speech over ordinary communication channels, and was further limited, unless the channel was secured throughout, to conventionalized code messages such as the one I had sent ordering the first two shipments of masters. Oh, no doubt the masters could communicate ship-to-ship and probably ship-to-home-base, but there was no ship nearby; this city had been stormed as a prize-of-opportunity, as a direct result of my raid on Des Moines in my previous life.
Such communication through servants was almost certainly not adequate to the purposes of the masters; they seemed to need frequent direct body-to-body conference to coordinate their actions. I am no expert in exotic psychologies; some of those who are maintain that the parasites are not discrete individuals, but cells of a larger organism, in which case-but why go on? They seemed to need direct-contact conferences.
He also suggests that each slug may be a cell of a larger organism. From chapter nineteen:
They did do some direct experimentation which raised my opinion of them a little. Vargas ordered brought in a baboon who was wearing a slug and had him introduced into the cage with the gibbons and the chimps. Up to then the gibbons had been acting like gibbons, grooming each other and such, except that they seemed rather quiet-and kept a sharp eye on our movements. As soon as the newcomer was dumped in they gathered in a ring facing outwards and went into direct conference, slug to slug. McIlvaine jabbed his finger excitedly at them. "You see? You see? Conference is not for reproduction, but for exchange of memory. The organism, temporarily divided, has now re-identified itself."
I could have told him the same thing without the double talk; a master who has been out of touch always gets into direct conference as soon as possible.
While he doesn't specifically state how the decision to grow an armored carapace was made, it is reasonable to suppose that it was made by the slug super-organism. It may have exchanged memory slowly, but it did learn and adapt.