Hey there,
Firstly: I took a look through a fair number of cards, but take everything I say with a grain of salt. I started playing magic again in August after a haitus since about 2008, so I don't have anything Modern memorized and I'm just starting to learn the new dynamics.
To start, it looks like you're going for placing counters on a lot of different creatures as opposed to focusing on a few. Therefore, I'd recommend pulling the Hardened Scales.
Creature cards you may want to consider are
Ainok (for first strike) and
Mer-Ek (for deathtouch).
Phalanx Leader can be used in combination with Feat of Resistance (below) or
Dragonscale Boon to keep the counters coming.
Instants:
Feat of Resistance. I utilize these as self defense; if someone tries to target one one of your creatures with any kind of spell or ability, simply give them protection from one of the colors in that spell. Your creature gets a counter, lives to tell the tale, and any spell an opponent used is effectively countered. I've saved Anafenza quite a few times in my drafts this way; she tends to pull a lot of aggression.
Planeswalkers: I like Ajani, Mentor of Heroes far better than Steadfast in this instance. It's true that Steadfast seems to play well off of Sorin, but with only 1 Sorin the deck and only 2 Steadfast, the chances of getting them together are low. Assuming you did, and very early in the game, this casting them both is 4 mana and 5 mana in respective turns that cannot be used to cast outlast/counter producing spells, etc. My experience with these decks is that there is always something you can do with your mana every turn, so I want as much of it to focus on my main deck theme as possible.
Enchantment: I highly recommend
Brave the Sands. I haven't had the opportunity to use it myself yet, but it has been against me several times and it been a game-changer time and time again. With outlast, you can attack with your creatures first. This leaves them open for blocking or outlast (if your characters have it). Should you draw one of those handy elephants that untap countered creatures during upkeeps, you can attack, outlast, then they untap for blocking. And if they are blocking, you have doubled the amount of creatures you can block.
I hope this helps some! And, as always, the best way to test if your deck is effective is to print out proxies and play casual matches until you've found a setup that plays the way you want it against many decks.