Yes, you only need enough to fill the voids between the transistor, insulator and heatsink. But unless they are unusual you dont need (want) that with sil pads. Read the docs that refer to the parts you are using, this is all known stuff, so read the literature. The danger of overtighting is not so much damaging the transistors, as reducing coupling by causing them to bow and lift up at the edges. Also you may cut through the pads with too much torque.
I use a torque screwdriver to prevent too much torque, not too little.