I may need the model to do further debugging. In the meantime, I created a simple FFT model in R2018b which seems to do the right thing in terms of datatypes. One change from your model is that the values are sent in serially to the FFT rather than all at once. I have also attached a model created in 15b.
I defined the input fftIn as:
fftIn = complex(rand(128,1),rand(128,1));