In case this is a pure junction between two materials, one should really consider how the atoms rearrange to form the interface. Sometimes it can be ok to make a highly simplified model, to make it possible to run the calculations with lower memory and time requirement, but the farther away the model is from reality, the less relevant the results become.
The number of screening layers is always a matter of convergence, for all system types. However, there shouldn't be an increased need for screening because of any tricks going on in X/Y, at least I don't think so.