Angels will get massacred by African cichlids eventually
way too slow moving. I would rehome the oscar as well, as they are a giant waste producer and require quite large tanks themselves. They do not mesh with Africans, either. Which Africans did you get? Did you buy them from the dreaded assorted cichlid section?
"The white patch is still on my other Pleco's head"-no expert, but that sounds like ammonia burning to me. Or some sort of surface bacterial infection
I agree that water changes and monitoring are your best bet, here. I hope I wasn't too negative with my above post. I had some beautiful angels get torn apart by my Malawi cichlids, and would hate for you to have that happen further down the road.