I imagine Tariq would try to write at least semi-formally but pepper his letter with slang regardless, because he doesn't have a great understanding of formal registers. Then again, he has Trudi as a sister, so I may be wrong. Maybe he gets Trudi to check his Celestia letters over for him.

I think Josh needs Twilight Sparkle as a pony friend, actually; she seems the pony best suited to teach him to stand up for himself and resist peer pressure. The problem is that Josh is drawn to full-on and intense characters; canonically, he befriends the Rainbow Dashes of the world and then comes to regret it.

I now really want to read about a Josh-Twilight friendship, to be honest.