You line
i->roulette = (int)(((i->fitness / sum_fitness)*100000) + 0.5);
is casting the value to int which is why any float operation is truncated try
i->roulette = (((i->fitness / sum_fitness)*100000) + 0.5);
and make sure that either 'sum_fitness' or 'i->fitness' is of of a float or double type to make the division a floating point division -- if they are not you will need to cast one of them before dividing, like this
i->roulette = (((i->fitness / (double)sum_fitness)*100000) + 0.5);
If you want to make this as a integer calculation you could also try to change the order of the division and multiplication, like
i->roulette = ( i->fitness *100000) / sum_fitness;
which would work as long as you don't get any integer overflow, which in your case would occur only if fitness risk to be above 2000000.