When it come to multi gene animals, the more the merrier. In your case it all depends on how much you want to spend and what your breeding goals are. Really consider your breeding goals short and long term before you start buying any animals. That being said, if you are considering breeding there is no need to buy any males for the time being, stock up on your females as they take a minimum of 18 months and 1500g + (I like them big) before they are ready to breed where as males only need about 6 months before they are ready to go. Powerhouse males are good to have in your collection as one male can breed with multiple females, thus increasing your odds across multiple clutches.
And yes, you are correct. If you had a Pastel x Bumble Bee you could produce Super Pastels.
Go to the World Of Ball Pythons Genetic Wizard and play around with all of the combos and potential outcomes!
Living in Canada, I have not dealt with Constrictors Unlimited, Royal Constrictors, or J. Kobylka Reptiles, however they are all very reputable breeders.