I know from the Yeerks' experiences with the Hawjabran that the Yeerks can only take as hosts species that have a central, unified nervous system.
But are there any other limitations?
Does the organism have to have a certain amount of intelligence? I know that the Gedds had limited intelligence, but they could still be taken as hosts. Is there a lower limit? Are Gedds the "dumbest" creature they have taken? (I know this is subjective.) What about a human with Down Syndrome, or an even worse mental disability?
What about physical brain size? Does the brain have to be a certain size for the Yeerk to "fit" in there? What about body size in general?
Does age matter? I know that the Yeerks infested human children, but how young? This also tells us something about #1 and #2, as children have physically smaller bodies and brains and are generally less intelligent (in many ways, not all) than adults.
Are there any other criteria for organisms that the Yeerks could successfully take as hosts? (I mean what they could do, not what they would necessarily want to do.)