Gold nuggets will get up to about 8" SL (standard length). They do grow slowly and are not the easiest to keep long term. They are herbivores by nature and need a lot of plant material to eat. They also like warm, soft water with strong current.
I have been pretty lucky with my group of 3. I have had them for about 2.5 years and they have put on maybe an inch or two but they are all healthy and active. They are mildly aggressive towards each other and chase each other around but no serious damage.
Most gold nuggets die from slow starvation as they lose the beneficial gut bacteria they need to digest their food. If you have had yours for 6 months and the stomach does not look concave you should be in good shape. But they still aren't easy to keep long term.
I feed mine Repashy gel food, Aqueon algae wafers and pellets, and sliced cucumber which they devour. I would not keep them with Africans as they like soft water but as long as they are feeding well I would not be too concerned.