Since city is captured it's safe to assume area around it is improved which will provide fast growth to city and it will become "profitable" with happiness. Only Ghandi's cities can be profitable with happiness. Besides if it's not stone age Ghandi's own cities undoubtfully will be larger that 8, it's Ghandi after all so the excess of happiness will cover those pathetic -4 extra and less happiness for new city.As far as I know the math goes like this: On a pop 9 city the regular civ has a -4 hit for founding a city plus -8 for population = -12 (Population 1 is free). For Gandhi a 9 pop city would have -8 for founding a city plus -4 for population = -12. This makes a pop 9 city the break even point on prince. This means the city taken would have to be pop 18 or higher before capture. I am not sure where you got pop 6 being the break even point from.
regular civ: -4 founding + -5 for pop = -9 on a pop 6 city
Gandhi: -8 founding + -3 for pop = -11 on a pop 6 city.
If my numbers are wrong it's due to a game concept being wrong and definitely not math.