
Going back to the issue of your dogs weight and diet... she doesnt sound underweight to me, she sounds fine.
Ignore what is said about how much a pup should weigh at what age, aside from the fact that all puppies are different and will put on weight at different rates, yours SOUNDS fine, she isnt fat, she isnt scrawny, so thats all good.
Bearing in mind her unknown origins, she may not be 100% GR, or she may be, but be bred from a rather small line... somethign ive seen a lot in pups coming from the two well known North West based puppy shops (think Samoyeds that look more like german spitz, tiny yorkies, cockers the same size as small jrt's etc)....
Her poo eating may well be that she is used to more food and is hungry- or it may be that her food is just too tasty second time round, which would hint that it contains things that are not being digested fully.
Have a search on here about the many kinds of foods and also the barf or raw feeding method, theres somethign to suit everyone.
Again bearing in mind her background, she now needs the best possible of everything - keep her lean for a start, if other dog walkers say shes a bit slim then in fact she is probably FINE (because most people are used to fat dogs unfortunately), if she has poor hips and elbows then keeping her lean will be the best for her.
Keep her exercise restricted - no galloping up and down stairs, or endless haring about with other dogs, limited on lead walking, you can exercise her brain by doing lots of little training sessions (look up clicker training and reward based training on here and on the net)...limiting her exercise in this way until she is fully grown (increase it gradually as she ggrows) will also minimise stress on the joints.
Get her out and about socialising now... never mind about the vaccinations, yes theres a risk, but if you stay sensible, dont put her on the floor in an area frequented by a lot of unknown dogs etc, but carry her around, get her meeting people, dogs you know to be haelthy, buses, cars, noisy roads, everything and anything you can think of, and make it REALLY rewarding for her, you can really limit future problems.
When she is safe to go out and meet other dogs, get her off lead asap, she will still want to be near you whilst shes small so take advantage of that to teach her a solid recall NOW - and also teach her how to meet other dogs nicely.
Em