I see the dobe also..... slimmer build with the ears....were they positive about the rotti ??I actually see more Dobe than Rott---her ears aren't quite Rott-ish and she has a slimmer build. It'll be easier to tell when she gets older.
But unless she's severely malnourished and underweight (and she doesn't look to be bad off), she must be mixed with something smaller. My Lab was 8 pounds at 6 weeks, and she got to be 55 pounds. So if your pup is 6 pounds at 8 weeks she'll probably be smaller than that. . .maybe 40-45 pounds.
I don't see Rott either. I've seen many spaniel mixes with this coat type and color pattern and ear set - when bred to terriers and even chihuahuas.She could be a mix of a minpin and something slightly larger. I don't really have a guess but I've seen a ton of smallish 20lb mix-breed dogs at work that looked a lot like your puppy.