Check ammonia, nitrites and nitrates with a liquid test kit. That's the only way to know where you stand with your water and if it's safe to add more fish.
Not sure how big PBass get but an Oscar will need a bare minimum of 55 gallons to himself. So it'll depend on how big a PBass gets and how big the tank is I'd say. Oh, and whether you can find Loaches 4" or bigger. An Oscar will outgrow Loaches by leaps and bounds and have them for dinner when he does.
Edit: Oops you posted while I was posting. I see it's a 75 g and they'll be going into a 125 g? You should have room for a school of at least 6 Loaches if the PBass stays under 12" (I think they do, not sure).