I agree. Rules out all the aggressive, glass banging male cichlids which need around 180G+ to thrive. If you used your 125G for a single fish instead, you could theoretically put in a female dovii, or umbee, or butti, as they are smaller than males, but it would be pretty borderline and they wouldn't be as aggressive.
Personally, if I was stuck with a single wet pet in a 75G and you didn't want an oscar (my first choice), I would go with a single nice male green terror (rivulatus). They are aggressive but not overly so, but they are also gorgeous to look at, and would be a perfect wet pet for a 75G.