Have to disagree with the GT in a 10 gallon, these fish can and do get bigger than 6", quite easily. They are slow growers when compared to other cichlids that people would group them with eg. oscars, texas etc. but they still get bigger than a 10G can accomodate.
Here is just one image of a GT over 6" (granted its technically a goldsaum, but this is commonly sold as a GT
EDIT: If you want a predatory fish, the true amazon leaf fish will fit that, but its not aggressive and can be tricky to keep, but with planning it should be do-able.
Here is just one image of a GT over 6" (granted its technically a goldsaum, but this is commonly sold as a GT

EDIT: If you want a predatory fish, the true amazon leaf fish will fit that, but its not aggressive and can be tricky to keep, but with planning it should be do-able.