Just two of their slaves would be enough to kill the other five. The reason they brought five reinforcements is to finish the fight as quickly as possible and prevent any accidents from happening.
So the four slaves they brought with them was already them over-preparing and overestimating the defense of the energy spring.
As for their other slaves, they left them outside the barrier to monitor the situation and be useful in other ways.
For example, they needed someone to make sure that the other end of the gate that they carried into the barrier was functional so that they would be able to escape if something went wrong.
They had other plans that they hoped not to use. All in all, it can be said that they are very prepared.
Their plans were not perfect, but they ensured that they would be able to survive this endeavor if something went wrong. So it was with confidence that they entered the barrier.
