For now, setting them all to additive let me combine them in a Blendspace without the body parts changing scale. In the Anim Graph I had to make sure though that I also applied the anim data as additive (otherwise no anims got played at all).
Montages should still be non-additive, so if I play a death montage, it plays only that and doesn’t blend with the idle anim.
Would still be interesting to see a solution on this! Guess I’ll try asking MalberS!