Let me give you a quick answer now, and perhaps I can tie up any loose ends later. One, starch and cellulose are both polymers of glucose; therefore, they are isomers of each other. The chief (but not the only) difference is in the stereochemistry of the acetal bond that connects C-1 of one glucosyl group to C-4 of another. Two, starch is definitely not a hydrocarbon and it is a storage polymer, whereas cellulose is the main component of plant cell walls.
Three, it is a necessary (but not a sufficient) condition of a molecule to have the formula Cn(H2O)n to be a monosaccharide. But polysaccharides do not conform exactly to this formula, because they lose one water for each acetal bond they form. Metzler defines a carbohydrate as being either a mono- or a polysaccharide, and that works for me.