Okay I actually have two questions.
1. Are all the bakeries and decorators making there cakes from scratch that you know of or is it okay to make a cake using cake mix but add a little more of your extra something is it still considered a scratch cake?
2. I have a book for cupcakes but I like the receipes and was wondering how I could use cake pans instead of cupcake pans. Would I just double the receipe?
Question 1 - is opening up a can of worms on here. This is a touchy subject on CC. So if scratch cakes work for you fine, but if box cakes for you that's fine as well. It's basically what works the best for you.
Question 2 - I have done that before. I just mix like you would for cupcakes and bake in regular pans. It usually takes a bit longer to bake, but just keep an eye out on it.
QUESTION 1: A scratch cake is made w/o a box mix. And yes it is def up to the decorator. One isn't better than the other. I have never worked in a bakery but from what I've read on here, grocery store "bakeries" usually have their cakes shipped to them premade & frozen so who knows. If you are using a box mix and then adding extra it isn't considered a scratch cake but again, it's up to the decorator. There is no shame in non-scratch or scratch as long as it's yummy yummy.
QUESTION 2: As long as you fill the pan the proper amount then there is no problem using a cupcake recipe. Remember cupcakes are just the kid size of cakes. Remember to adjust your cooking time as well.
I have worked in a bakery and the vast majority use 50# bags of mixes. Reason? Bakery employees aren't paid a lot, so the Bakery has employee turnover. To get product consistency, they use mixes.
If you're a one-woman show like me, I'm the only one baking, so I can get consistent results baking completely from scratch.
I disagree with you on one being better than the other, but hey, that's my opinion. Which along with $3 will get you a cup of coffee.
If you do a search, you will see that this question has been discussed (heatedly) repeatedly.
I'll answer the doubling the cake recipe question.
Be careful. Not all recipes (baking and cooking) benefit from just doubling the ingredients. In baking you could be adding too much leavening if you double the recipe. I am finding that most cake recipes are over leavened to begin with. This is probably to keep the tops flat, at least that is what I have read so far. I have been doing baking/science experiments playing with the leavening to make it proportional. I've been doing this on cupcakes only, haven't tried a whole cake yet. I find I like the look of the proportionally leavened cupcakes better than the overleavened.
As for the cake mix vs. scratch - I have no idea.