No, sheeps milk doesnt always mean the same thing for goats milk. Dairy goats actually have a lot of similarities to a dairy cow remember.
I agree adults can get cocci. As the animal gets older, it develops resistance to cocci, so therefore in theory adults should be fine. However, in practice there seem to be different strains of cocci, and an adult might be immune to the cocci you have on your land, but if moved to another holding, can then fall to a slightly varient of cocci. How long have you had her? if less tahn 6 months it is entirely possible that it is cocci, and this is her just displaying symtoms now. If it was listeria, I wouldnt expect her to be improving yet, and from what you say, she sounds a little better.
Do you treat for fluke? I have had goats that scoured quite badly as one of the first symptoms of fluke damage....
My mother (qualified vet) would always say you have clear up the scour first anyway before you treat the cocci/fluke/worms as doing otherwise can just overload their system and even kill them.....
Beth