First things first, it seems as though it's not so much harming the dogs, but you. You say it scares you when the Lab plays with the Pap. Leave them to it! Your girl will stand up for herself if the Lab gets too carried away. Obviously stay and supervise the dogs while they're playing, because larger dogs can do allot of damage if they have the time.
Have you tried, putting li'l Pap in her crate, and then putting food down for the Lab so Pap can see? I've never had any food aggression problems, so I'm not sure if it would make it better or worse. Another option is keeping an eye on Pap when she eats. If she shows any sign of aggression toward Lab, pull her away from the food. I know that one will work, whether it works right off or you have to wait a year. :mrgreen:
Try walking them together, training them together, put their crates side by side. When you're stroking the Pap and the Lab comes over budging his way in, push him away, the same goes for if you're petting him and she goes over. Remember to call the guilty-of-attention pup back over once you've finished razzling with the other.
She has been a single dog for as long as she can remember, why would she accept a new dog? It's like putting a toddler who' never lived with another toddler in the same room and expecting them to drink from the same bottle. You've basically lodged this dog, and your Pap mix isn't liking it right off. Because you're the pack leader, you decide what happens in the house. Make sure to keep everything tight in order for the next few days.
Hope this helps.