gliders can have any kind of apple including crab apples, though crab apples are usually only palatable if cooked.
Peggy may have been specific about the apple for her salad because that particular apple calculated in the best for her salads, I'm not sure tho. substitutions are often made with her salad ingredients so it's hard to say without asking her directly.
Apples are part of bml no specific apple is listed for it.