When attacking a Horde in Deathwatch, do you apply both the Horde size modifier and the size modifier applicable to the individual creatures? Or does the Horde size modifier replace the creature size modifier?
The size bonus is replaced by the bonus dependent on the Horde's magnitude. The normal size bonus for individual creatures are not cumulatively added together (Deathwatch Core Rulebook; Pg. 359; “Attacking A Horde”):
Characters must still roll to hit a Horde, but the appropriate size bonus should apply to these tests based on the Horde’s Magnitude.
Now this doesn't mean you have to stick to this rule, especially if the horde comprises mostly of obviously larger opponents. Do not feel as if you're forbidden from increasing the modifier up a notch or two to account for how much easier it is to actually hit such a horde.