I have two BGKs... one 10", the other 12"... Both were raised from 4" with Oscars, Dempseys and other Cichlids... Both still live with Oscars, Dempseys and other Cichlids...
The only aggression issues I've had with BGKs (giving or receiving) is when I have breeding Cichlids in the tank. My BGKs seem uncontrollably addicted to eating eggs/fry and savagely attack Cichlid nests fighting both parents to get to the eggs/fry. Initially I was shocked at the ability of my BGKs to fight both parents at once and still feast on eggs/fry.
Be careful who you listen to. Nothing is more valuable in this hobby than listening to someone with experience on a particular topic... and nothing is more worthless than listening to someones speculation without experience...
I do need to add that my experience with raising BGKs with Cichlids has taken place in adequate sized tanks that are not overstocked. I would not consider a 29 gal tank adequate for Oscars or BGKs.