If you are going to change her food, I would wait a few weeks to give her a chance to settle in - change of home and all that involves is enough change for a new puppy for a while and as you say, they can have very sensitive tums
When Coco was very little there was no end to the things that would give her a runny tummy.
Re treats: if you are using kibble it is a great idea to use part of her food allowance as training treats. Otherwise, Coachies are good and not full of additives as are the Nature's menu treats (I cut them in half) - otherwise, small bits of cooked chicken or ham and, as you say carrot and apple (if she likes them - Coco doesn't
) Also, you can easily also make your own healthy treats - see this thread for some ideas:
http://www.cockersonline.co.uk/discuss/index.php?topic=30234.0I crisp up the livercake and tuna bread in the oven and then they keep longer and I also put small bags of them in the freezer so I always have a supply.
Hope this helps. Bet you are excited - get plenty of sleep tonight and have fun with Bella and remember, we need lots of photos