i always trust nature and the mamma to do the right thing, and it usually works. we have had great luck having our hens raise our chicks...and it is so much less work for me. if the other hens get too aggressive, you could always take the mama and the peeps into their own space. the mama teaches the chicks to forage, keeps them warm, teaches them social skills, and so much more that they miss out on if you take them away from her.
but, are you ordering your speckled sussex peeps, or is one of your hens hatching them? if you have a hen hatching them, she will most likely care for them, but if you are ordering peeps and putting them in with your hens, unless they are already broody, they most likely won't care for them.