Hi Veres,
Is it ideal to use one ISO for each simulation? For instance, should we shoot at 64 for Kodachrome 64, 400 for Portra. Or does it matter? If it does matter what ISO setting should we use when they are not an available when the Gamma is set to a particular setting?
Not really, you can use whatever values you feel are needed to get the proper exposure.
Of course the best ISO value with most benefits is the first base ISO, because it offers the most dynamic range & lowest noise possible.
The only time when ISO matters is when using Black n White Film. By turning Off the Noise reduction and by adjusting the ISO values, we can increase or decrease the amount of grain present in the picture when using on Black & White film simulations. Besides that just try to use ISO like you would normally.