But are the Chee sentient or just very cleverly programmed? I'd say no, Chee cannot be assimilated. That'd be like the One absorbing a ship or a building.
I think he can only take biological mass and absorb it. Sort of like morphing, but instead of becoming the animal, the animal becoming another part of you. Constantly there, slave to a hive mind where the One is the queen and physically part of his biological mass. The animorphs can't morph objects. (except clothing which I always had a problem with. They shouldn't be able to do that, though it was convenient)
I also, sorry Chimi, don't believe he works on the 4th dimensional(?) level.
TBH, I personally always thought that the worl-emcompassing creature, "Father", from the Ellimist Chronicles had to have come from somewhere. Perhaps spores shoot out from a planet or decend froman asteroid and where those land, a creature grows and takes the planet if they are not stopped.
I believe that The One is one of those creatures who is unaware that he is not the only of his kind and has managed to break free from his planet. On the way, he gained the morphing ability either from an escafil device or a naturally morphing alien or something.