yeah, but the problem in #18 wasn't with the size of the morph, it was the fact that a ship happened to be cruising through Zero Space exactly where the technology decided to dump their extruded mass. They've morphed fleas A LOT, which was fine, and I think they even morphed mosquito again later with no ill effects other than the mildly disgusting fact that they'd fed on their friends.
I really don't see why they couldn't morph bacteria. Just like I don't see why they couldn't morph mushrooms or daisies or sea sponges. All comprised of DNA. And yet, when you think about this, it doesn't feel like it should jibe, right? I mean they get the animal's mind and instincts when they morph...what would an Oak Tree have to offer? Would they even maintain the mental cognizance to be able to demorph?
For fic writing purposes, I decided kind of arbitrarily that morphing is limited to CNS-based organisms. There has to be some kind of nervous system to support the limited, yet present, "mind" so that the morpher can actually *use* the morph.