I think it depends on skill. Founders can replace people with a degree of detail that fools medical tricorders. I don't imagine the components of a communicator are much more or less difficult than a complicated nervous system.
But fooling a medical tricorder does not necessarily mean becoming completely and convincingly human. It merely means fooling the device. If a Founder did become completely and convincingly human, it would cease to be a Founder and could never shapeshift back!
No doubt the Founders to a large part rely on sleighs-of-goo to confuse the observers and instruments around them. Their "authenticity" could be skin deep, perhaps with a sensor-impermeable barrier just below skin depth and a series of special organs emitting false signals of interior structure and composition.
Good point, Laas went big-to-small in space, so your theory would work to explain it.
Would a Changeling stranded in deep space so callously discard his raw materials, though?
Remember as well that Odo was still a changeling in early S5 - the Founders forced him into a specific form and prevented him overriding the lock (until the dying infant in The Begotten undid it), but they didn't bodyswap him into a human body or anything of the sort. And in that form, his blood stayed blood when removed, he had internal organs, tastebuds, etc...
...An ability an adult Founder would probably have to possess, in order to convincingly masquerade as General Martok or Colonel Lovok for months or years at an end.
Probably the 18-hour regeneration cycle of Odo's is more akin to the need of a baby to have naps every few hours. A baby will cry a lot if deprived of this, but an adult won't mind if he only gets to sleep once per day. Except with the Founders, it's probably even more extreme: an adult Founder doesn't need a 50-hour cycle versus an 18-hour one, but more like a 50-month cycle. Remember that the Female Founder did essentially just fine despite being deprived of shapeshifting for months upon months at an end.
So again I'd like to draw a very sharp distinction between what a Founder can do, and what an unstimulated child Founder possesses the skills, patience and experience for doing.
As for Odo's communicator, maybe he has the ability to absorb into his body inanimate objects, making them temporarily part of his liquid state, then able to reform it when needed. Or perhaps he shunts inanimate objects into this same dimension where he can store or pull out extra mass.
Both of those are IMHO preferable to the idea that Odo re-creates the complex device every time. An adult Founder might be capable of this, but Odo should lack the skill, considering what other skills he lacks.
Or maybe the Great Link itself is extradimensional, and when the shapeshifters lose mass, it shunts back to the Great Link. When they turn into a larger form, it pulls some of that metamorphic liquid from the shared pool of the Link.
Now that's original, and quite intriguing! One wonders what sort of things could be transmitted through this shared realm... Apparently not the S31 disease, at least.
Timo Saloniemi