Congratulations
Not every pupppy has a sensitive stomach. It makes sense to swap diets gradually over a few days because it is the main source of nutrients, but I wouldn't personally worry about treats. Using a portion of the daily food ration for treats is a helpful trick both for sensitive stomachs and for cockers that are prone to overweight. But you will probably need some treats of higher value than regular food too. Most puppies are fine getting different treats, you'll notice when you go for vaccinations that the vets bribe puppies with treats without worrying about what they are used to eating. That said, every puppy is different and some are more sensitive than others. But my puppy came from a breeder that fed a cheaper type diet mixed with boiled vegetables. The dogs got pears and apples for treats. I have continued giving fruit, vegetables, fish, meat and eggs in addition to a quality diet. I think variety is good for most dogs and puppies as long as they have quality base nutrition and don't have too many new things at once. I guess I'm not giving a clear answer. But it really depends on the puppy how much change and treats they can handle without getting tummy upset. Mistakes will be made and it is usually going to be ok. At 5 months old my puppy gulped down half a serving of spicy curry from my favourite indian restaurant. I was horrified. She never as much as burped, no tummy upset, nothing
Some puppies are sensitive to change, some can eat pretty much anything.