This is actually a *huge* argument in the fandom that someone else can probably discuss in greater detail than me, but the short of it is there's nothing really in the text that describes them as having humanlike shoulders or even a torso at all, and not much in the way of explicitly describing them as centaur-like either. In fact, some characters describe them as resembling blue deer at a distance, which is a real stretch if they've got human-esque upper halves.
All of the official art does portray them as centaurs though, but that could have been just artists making assumptions and Applegate couldn't do anything to change it after the first official art came out.
Basically the short answer is 
Not to go all "sharks aren't smooth" on you, but I think we can pretty officially and conclusively put this to rest. There are one or two descriptions that are ambiguous enough to cause confusion, but the text pretty clearly backs up the official artwork in lots of places. I took a sampling of expository 'Andalite appearance descriptions' from a few of the earlier books:
Finding Ax in book #4: He stood on four delicate hooves, looking, at first glance, like a pale blue and tan deer or antelope. But he had a
strong upper body, like a mythical centaur, with two small arms and many-fingered hands. His face was almost triangular, built around two huge, almond-shaped eyes. There was a small vertical slit where his nose should have been, and nothing where his mouth should have been.
From book #6: It's amazing how quickly we'd all gotten used to the fact that this guy from another planet was with us. I barely even thought about the fact that an Andalite was standing there, looking like a cross between a blue deer,
a mouthless human, a goat with eyes on the ends of his horns, and a scorpion.
From book #7: He looks like some odd cross of a human, a deer, and a scorpion. But not really like any of those things.
His upper body and head are more or less the human-looking parts. He has thin arms and many-fingered hands. His face is flat, with slits for a nose and two large almond eyes. He has no mouth at all, which is why thought-speak is the natural language of Andalites.
From book #8, when Ax is describing himself: I am told that I look like a cross between a deer, a scorpion,
and a human. I've seen deer in the woods, and I don't agree. For one thing, they have mouths and I don't. And they have only two eyes, while I have four.
From book #9: Up close he looks like a cross between a deer or a horse, and a human and a scorpion.
Sort of like a mythical centaur. His upper body is like a boy's. He has weak-looking arms and a head with two movable stalks on top, kind of like antlers. Each stalk has an eye. The eyes are constantly looking left and right and back.
I got tired of looking after this

but I'm sure there are more. Alt-Andalite designs are fun though! But I think the text does pretty much back up the centaur comparison.