
I've had 2 litters of singletons (from the same Mum).First was a huge dog pup, who grew into a huge dog,second was a correct size bitch pup who grew into a normal size bitch. Mum was a bit over-protective of the dog but when she had the bitch she was more laid back. I think they do get a bit lonely and do need more toys and more attention from their owners, especially if Mum isn't particularly playful with them, but it's not so bad if you have other dogs who will tolerate bored puppies!
I (or rather the Mum)had no problems with milk supply. They usually produce to demand, so as long as one puppy is feeding, they produce what is needed and no more. If the bitch is likely to be susceptible to mastitis, you probably do need to keep an eye out for it though (I'm sure you would anyway!). They were both extremely well fed pups - at one stage the bitch was square but was up and about normally and neither were fat adults.
Mine didn't have any more or less socialisation than any other litter and didn't have any problem with other dogs when they matured.
The upside of a singleton is that they get eveyone's undivided attention. The downside is that's it's almost impossible let them go! (Mine stayed for 15 & 14 yrs respectively ......)
Good Luck, I'm sure she'll be fine.