1) I always tell my new pup owners to allow their pup to mix with other dogs, so long as they know they are fit and healthy, up to date with vaccinations and that they are meeting on 'safe' ground, ie a garden where unknown dogs don't have access. Very important that she continues with the dog-dog socialisation.
2) Shouldn't worry about the runny poos, unless she shows signs of being unwell. There are a lot of changes to her life at the moment, including water (a change in water can be a very big cause of runny poos) so it's quite natural for her to have a bit of a dodgy tum at the moment. Please do not change the food your breeder has recommended (unless it's out and out rubbish), that's the first thing many people do. The food has suited her at the breeder for all this time, it's not going to suddenly upset her in your house ;-)
3) For goodness sake don't buy treats to give her for training ... mostly highly processed junk. Yuk, yuk, yuk! I'd use some of her puppy food from her daily allowance and fresh meats, cooked or raw, depending on what you're feeding her with. Forget the Pedigree c*** though :-( ... If you get her playing with a favourite toy, a tuggy say (my preferred interactive toy), then this is perfect, specially for recall training and reward for toiletting in the correct place. I never use food for these. Depends on you and your dog though.
4) You could buy yourself a noise desensitisation CD - obviously not been used by your breeder. I play one of these from the moment the pup's ears open, very quietly. By the time they're 8 weeks they're listening to it at full volume and couldn't give a hoot. I've done this with a pup I got at 10 weeks of age, he went on to be a very rock solid with noises search and rescue dog :-D
This is the CD I use:
http://www.companyofanimals.co.uk/products/clix/clix-puppy/clix-noises-and-sounds-cdIf you are willing to do it, it is very effective. Must be done every day, twice a day. Start off at a volume you can barely hear, play for a few days then increase the volume very slightly. You're looking at being at full volume after 6-8 weeks.
Taking her into a village/town and sitting on a bench watching the world go by is also very good. If she's frightened start off somewhere a bit quieter. Supermarket car parks are excellent, specially if she's bothered by cars. She can watch them pull up and then for the nice people inside of them get out and make a lovely fuss of her. A little bit at a time and often. Remember, all experiences must be positive at this age ... just build things up, and exposure to more and more things gradually, never out face her.
Good luck with her, there's so much fun with pups of this age. :-D