A few days ago while Alice was trying to understand her abilities better. She was trying to understand how to get more familiars. Mia and Robin were her only familiars. Their case was pretty different.

Soul servants are more or less expendables. They don't have specific powers or abilities. They are like soldiers that will learn from experience.

Another difference is Soul Servants consume Chaotic energy. While on the other hand, her familiars consume mana.