I would strongly recommend the Cycle of Arawn. The only of your points it comes near to breaking is the first, and even there it's less that the protagonists are assassin's, and more that they're adventurers who happen to be able to assassinate people... So sometimes they do.
As for the gyst of what it's about, it's mainly about the adventures of two teenagers(Though by the third book, they're both in their mid to late twenties.) Who are the main characters.
The first is Dante Galand. An ambitious, somewhat monomaniacal, kinda jerkish (and by kind of I mean incredibly.) Young man who wants to learn magic. So he steals the holy book of cult famed for their use of it only to freak out when cultists try to murder him and so he hires...
Blays Buckler. A Mercenary Armsman, slightly younger than Dante and so unable to find work with the better paying guilds despite his skill with the sword. Though he is a Mercenary, he's far and away the more moral of the two. Whereas Dante is willing to do basically anything that helps him achieve his goals, laudable though those goals often are, Blays very much is not.
Despite these conflicts, the two share a sort of adopted brotherhood after they've saved each other's lives a few times, which keeps them working together.
While not entirely formulaic, the author likes to expand the map whenever things get to feel too stale. Introducing new places, cultures, and mysteries as he goes. This isn't always necessarily a far flung locale either, sometimes he will just elaborate on some strange people that's geographically near, but culturally far.