Or you could just allow the use of any primaris model to be used, and if someone wants to cut it up then they have to. Otherwise it is quite limited with just the specific mentioned above.
at least with standard primaris they are a blank slate to convert.
The four listed above are the four affected by the 25% Rule and will need to be cut up to be used as a compliant entry, EVERY other Primaris model can use their torso/legs assembly (this INCLUDES LIEUTENANTS) unrestricted, as long as the final model LOOKS like the model is representing from the list, it doesn't matter where the parts come from.
To be absolutely clear, there are no models or bits outright banned from the challenge, there is only limitations on how much of some models one can use.