[CST] Ambiguity with Floquet modes simulation?

Hi everyone,
hope this topic has not been solved already (I don't think so).
Apologies in advance for this long post, but taking time explaining things may help to explain my issue.

I am currently working on the simulation of FSS transmission characteristics with the frequency domain solver of CST Microwave Studio.

From theory:

You can find that the onset frequency of grating lobes, i.e. Floquet modes cutoff, is defined by the relationship below :

fg = c/(D*(1+sin(θ))), where D is the periodicity of the cell (let's say its square here), c the speed of light and θ the angle of incidence of the impinging wave on the FSS you want to simulate.

While becomming propagative, each Floquet mode will couple some part of the energy and thus should be taken into account in the response of the structure.

In CST MS:

After setting the operating frequency range of my structure, I can define how many Floquet modes are necessary to simulate it properly going in Boundaries > Floquet Boundaries > Details. This way I access to the mode calculator which takes as inputs the maximum frequency of operation and angle of incidence considered, and computes the α and β propagation constants. Modes for which α is null mean that they are propagative in this case and should be considered in the computation.

Once the appropriate number of Floquet modes defined in Boundaries > Floquet Boundaries > Details, I assume the unit cell boundaries are correctly configured.

Then I come with my issue:

Going in the Frequency Domain Solver Parameters, you can access to the excitation settings of the simulation, choosing either the source (impinging wave from the front or the rear of the FSS), and the modes excited from each of them.

Here's my question :
Since I am only interested in the response of the 2 first propagative modes of the sources (TE(0,0) and TM(0,0)), can I choose to excite them only? Will all the superior Floquet modes be properly taken into account if they are defined as considered in boundary conditions but not in the excitation?