I agree with the initial statement made by your
LFS, " The Cories are better bottom cleaners than the Pictus."
IMO the Cories will hunt down some of the smallest morsels of uneaten food as they "graze" thru your substrate and plants, while the Pictus is going to be going after regular food and the larger scraps of leftover food.
The Pictus is a like a broom, sweeping up the big stuff off the substrate but the Cories are like little shop-vac's sucking it all up!
Once your Oscar is big enough, anything small enough to even begin to fit in its mouth is given the Ol'edibility taste test!
I would not, NOT!, put Cories in with the Oscar. They would become snacks at some point and could seriously hurt your Oscar if he trys to eat'em, the spines on their pectorals can cause serious injury to a young, curious Oscar's mouth-n-throat when trying to eat'em.
The Pictus cat has been used as a tankmate for Oscar's with varying degrees of success and failure. It will depend on your Oscar's temperament and general mood (Oscar's are subject to more mood swings than a woman suffering severe PMS
).
Imo opinion if your Oscar will tolerate'em then they would compliment the Oscar's messy feeding style rather well.