[Spoiled Strike] and [Shattering Impact] are flex slots. Keep in mind that attack speed and range do not affect hound weapons.
[Shattering Impact] is here to remove armor with [Diversified Denial].

Mod elementals however you like it. Elements modded here are just for more (gun) condition overload damage.
If you want your hound to kill more easily, it makes sense to go for viral heat/slash, corrosive heat or other faction advantage. In addition, a (Primed) Smite mod is much better than [Spoiled Strike]. However, this all requires extra forma.