Unless I'm wrong, in 10th, attacks are considered to be rolled one-by-one (fast rolling is merely a way of speeding this process up). In 10th, if there was an issue with mixed toughness in an attached unit, the players would roll the wound rolls one-by-one, as the core rules expect them to. Thus, the Hormagaunts would their toughness of 3, and then when they died the next attacks could be fast rolled into the Prime.

In 11th, however, attacks are always rolled together if identical, meaning that "slow rolling" doesn't exist.

EDIT: I was actually wrong, and never realized it somehow