Yes it's obligatory.
"חיות לא רעות" means "not bad animals" - it doesn't tell you something about all the animals
Yes. To express that a noun is an adjective, when the noun is indeffinite, you need some pronoun or זו/זאת/זה.
I think he meant direct object, for example "A cow is not a horse" would be (para he lo soos):
"פרה היא לא סוס."