1, Is it worth switching to weapons in the heat of an battle ? Or is it a suicide like in ut ?
Switching to a weapon to finish an opponent can
definitely pay off! Switching from the rocket launchet to shotgun or railgun can be very effective. Keep in mind which weapon from the three categories (1. projectile 2. hitscan (instant hit) 3. melee (weapon 1, gauntlet) is best to use.
2,Whats good practise for aiming? Im very used to moving while aiming and shooting from ut 2k4, yet in quake 3 I stop before I shoot from rail gun.
Stopping briefly, or even jumping before shooting the railgun is quite common. It depends on the distance between you and your target of course.
3, Like I noticed above, should I always move while aiming ?
Standing still at unexpected times can serve as a fake move, to put your opponent at the wrong foot, but it takes some time to know how and when to use this. Most of the time it's better to keep moving though.
4, What aiming is good in quake 3 with rail gun? Should aim with my mice, or strafe more?
Strafing to aim works well for all instant hit weapons (rail, shotgun, lightning gun, machine gun). For quick reflexes (flick shots) and carefully aimed shots (shooting someone in the air) you'll need precise mouse movement.
5, And finally, is it worth playing with bots, will they increase my aim?
If you can beat all nightmare bots, you can try to beat them all without dying once.

I
managed to do that some time ago and it's a good training to try to stay alive through all maps and still reach the frag limit before the bots do.
6, Oh and I forgot , whats a good way of dodging ? Jumps or just strafing ?
Strafing, because you can't change direction in the air. Jumping at the wrong time can be even more dangerous than standing still at the wrong time.
7, Last

, whats a quad jump ?
A quad jump is nothing actually.

You can use quad damage in combination with a weapon that gives knockback (rocket/grenade/BFG/plasma gun) and shoot at the ground just after jumping. But that's called a quad weapon jump (quad rocket jump for example).