Hi Mateo,
As you noticed the `SimpleInteractionFingerprint` are complex fingerprint, because they were intended to be used for similar conformers clustering, not for cross protein model training, as I understan your task.
There are also `InteractionFingerprint` which return 8bits per residue in a protein (some refer to it as PLIF). Since the number and type of residues are different across structures you cannot use them for your application. There are `SimpleInteractionFingerprint` which does what you ask - bins amino acid types together regardless on the position in sequence.
This is one of the exact reasons why we developed PLEC, and I hope they serve you well. you can simulate to some extent how SPLIF encodes interaction by setting both depths to 1: `PLEC(ligand, protein, depth_ligand=1, depth_protein=1)`. It will not be exactly the same, because SPLIF only has depth=1 environment, and PLEC will contain depth=0 too, but I would not think that is a big difference.