I think mostly they are just "shakes" from existing hives which gives you a mix of both....you will probably see a high attrition rate over the next few weeks....and the appearance of your bees may even change. Many breeders ship with Italians or whatever, but the Queen's offspring is what will make up the composition of your hive, and they are sometimes a different race of bees.
This is dead-on. I suppose that the time of day they do the shakes will determine the ratio. Middle of the day will miss a number of older field bees, giving a higher number of nurse bees (since they stay on the comb regardless). It will also depend on which frames they shake, since the the outer combs will typically have a preponderance of field bees, while the inner frames will be more nurse bees.