Yes you can keep them together but the pictus HAVE to be full grown before the oscar gets full grown otherwise it will try and eat it but the spines will cut into the oscars throat and get lodged. You will not be able to get it out without killing one or both fish.
my oscar doesn't bother with anything unless it looks food sized and swims at mid-level. He takes bites out of my goldie, but the goldie never learns to leave the oscar alone.
You can house pictus, just depends on the oscar itself. Most will not try and eat them. However if the oscar does attempt to, try dividing the tank for a few days.