I can answer your question about legato
The best way to get strength and speed in all your fingers is to practice your hammerons and offs like trills for ages using all your fingers. sooner or later you'll be able to run up and down scales just using hammer-ons and offs.
The 'cheat' that Satriani uses a lot is to your picking hand around the neck and strinks near the headstock to mute them while playing legato. I've seen Vai use this sometimes too.
Miracle Blade 4: Gibs on touch.