I have 5 juvenile oscars (in a 300 litre tank) that I bought at the same
time, all of a similar size, the idea being that a pair might form. They
have grown at different rates and now the two largest (4"?) which are almost
identical are dominating the tank hanging in mid-water at the front. The
others hide most of the time.

Before I return the other 3 to the shop can I be sure this is a pair? Have I
read somewhere about temporary alliances of sub-adult males? Should I wait?

Ade.