What is the weight range of an adult mini daschund? Is five to six pounds too small? What are the warnings on double dapple dogs?

I can't say, but the important thing is to keep your dog lean especially a growing puppy. Your dog definitely should be narrower at the waist than the hips and chest.
You should be able to easily feel the ribs, but not see them. Each dog is
Different. Standard recommendations are a good place to start, but each dog
Must have its food adjusted to its individual needs. Here is a link to a good illustrated guide,
LongLiveYourDog.com - Life Span Study - Rate Your Dog