How does it work together?
will it preselect the gun or select immediately?

First, some background on qw weapon handling, in case you are not aware of the mechanics. In most modern game modes, a weapon can only be dropped to the ground on death if it is actively selected (ie. held in your hands) at the moment you are killed. Modern weapon scripts for team deathmatch automatically hide weapons when not actively firing or reloading to minimise accidental drops to enemies.

The two sets of commands you reference don't really work together. b_switch and w_switch are very old commands from before the days of weapon scripting. They will automatically actively select higher-tier weapons when picked up (which is not considered desirable behaviour in any multiplayer mode), but will not enable them to be fired while cl_weaponpreselect is enabled. Once you attempt to attack with "cl_weaponpreselect 1", the preselected weapon will be fired instead. Both b_switch and w_switch are recommended to always be set to 1 in all game modes.

On the other hand, cl_weaponpreselect synergises well with cl_weaponhide. These modern commands are designed to prevent you from minimising weapon drops to enemies in team deathmatch modes where strong weapons are a scarce commodity. Using these will try to prevent weapons from being actively selected when outside the fire/reload window because they will "preselect" weapons, rather than actively select them. These commands are usually not considered desirable in duels where dropping weapons for yourself when you are out of map control is very helpful.

well, I'm used to play with w_switch 8 and b_switch 7 and no preselection/hiding,
sometimes in FFA it is useful to start firing as soon as possible once you got a gun, so I would like to keep w_switch and b_switch behavior and at the same time use this new possibility with auto-hiding.
Is it possible to achieve?

I was trying to get any working setup today and I FAILED.
See some bugs I found while trying to get this work: https://sourceforge.net/p/ezquake/bugs/539/