A Hoplarchus psittacus (true parrot cichlid) will outgrow the tank and they can be aggressive, especially in tight quarters. A max size Hypselecara temporalis (chocolate cichlid) would be pushing it in a 90 gal and some, if they don't eat plants, like to uproot plants. If plants aren't a priority, a Heros sp. rotkeil would be a good fit with geos and they're not one of the larger growing Heros, they tend to stay in the 8-9 inch range.
Geo tapajos don't necessarily stay 6 inches. Some might, but I bred them and 6 inches would be a minimum for my males, most were closer to 7 inches, occasionally up to 7.75 inches. A colored up group of Geo tapajos (along with a number of other geo species) can be their own centerpiece. I suppose it's all how you perceive it, but except in a larger tank where you could keep a showy 11 or 12 inch plus fish (like Hoplarchus psittacus), I think of additional fish with geos as more of a complementary piece than a centerpiece.
I've had my share of A. rivulatus (green terrors) over the years. Except for the wilds I had mine weren't especially aggressive. But they can vary. As for size, they occasionally reach 12" but much most are in the 8-10 inch range (males), so not as large as a couple of the other suggestions here. Not recommending them for your tank, but I wouldn't automatically rule them out. I think someone mentioned Andinoacara stalsbergi above, which is the other fish often called green terror. Stalsbergi are actually the hard water cousin to rivulatus, rivulatus in the wild inhabit more neutral waters.
Blue acara are variable in color depending on collection location or breeding strain. I've also had yellow acara (Aequidens species) with geos. I've had wild angelfish as complementary fish to geos at times, which works well, different shape, behavior and levels in the tank, but that might not be what you had in mind.
ryansmith83
has witnessed similar behaviours. 
