I think Joss also picked him to add more conflict to the group. Hopefully, no-one wants QS on the team and don't welcome him with open arms. He should feel forced upon the team and everyone is very reluctantly working with him and would like to get rid of him.
Everything that Joss wrote for Spike, he needs to ramp up even more for QS. He can't just be a likeable a**hole. He has to be a complete unrepentant *****e who comes off really arrogant before there is even any kind of character redemption later in the film. If there were ever any list of most *****ey movie characters, QS really needs to come near the top if not right at the top.
He should be at least as much of a *****e as Louis Litt (Rick Hoffman) in Suits, and then some. Don't go in half hearted.