Not being rude, but can I ask why it couldn't be permanent if they get along?
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It can be done, i just wouldnt recommend it. Have you ever seen a full grown oscar? They have a massive bioload. You would have agression issues and a VERY difficult time controlling nitrates. You might think you have seen a full grown 1 but that might only be like 12". Have you seen a 16"+ oscar? If a 75g is recommended for just 1 oscar by itself, imagine not even twice that gallonage yet 3x the bioload. Its up to you, i just dont think its a good idea.