I’ve got a little chick that had pasty butt so I washed it up to clear it’s vent but it looks like it possibly has a prolapse? Do I dare put it back with the other chicks or will they peck him to death?
If you want to keep it separate, you could try dividing your current brooder with a piece of wire mesh (hardware cloth or similar.) That would let the chick be "with" the others so it does not get lonely, but it would be safe from pecking. You would need to make sure both sides have food, water, a warm space, and plenty of cool space. Both sides also need enough room to move around.
I would probably try putting it back with the others and watch to see what happens, and only separate if that seems needed. Staying with the others is best for the chick if it does not get pecked, and that is probably easier for you as well.