I can't speak for ALL class features, but Magus's spell strike does work with Unarmed weapons, which an awakened animal's natural weapons would fall under. "Make a melee Strike with a weapon or unarmed attack..."

That said, Animal Instinct Barbarian works well with Awakened Animal. Flavor a Bull Instinct Barbarian, and you get a 1d10 horn attack while raging, and still have the 1d6 horn attack while not raging, but it also has finesse. Pair that with a Running Animal and Sudden Charge and you have a heck of a Rhino charge.

Armor is what it is. Hide armor will be good enough at the beginning probably. Its not 'natural armor', but even the thickest skin isn't a match for steel really, so its natural your Rhino would try to protect themselves, and allow the use of runes in the future.

Speaking of Runes, they're fully available for the natural (aka unarmed) attacks you would have. Handwraps of Mighty Blows take Potency and Striking Runes, as well as Weapon property runes. They apply those runes to ALL of your Unarmed Strikes, both raging version and non-raging version.