May I adjust, in the Planar 3, the cartridge force by only using the weight, leaving the traking force in zero ?Yes you can, but you'll need a stylus pressure gauge to set the tracking force.
What about the bias adjustment ? I also heard that in Rega RB300 is better to leave it in a lower value than the one indicated by the cartridge maker. On the contrary, Rega suggests to keep it in the upper limit...Bias adjustment on a cartridge? I don't understand the question. Are you talking about anti-skating? Or tracking force?