To make that violin sound , use the volume controle to remove the sound of the note attack ... Pick,hammer or pulloff a note with the volume controle iz zero , then increse the volume .
You should practice to controle the volume knob with your the picking hand's pinky finger , or use a volume pedal to make it easier (assign your zoom's exp pedal as a volume pedal).... Malmsteen uses the volume knob on his guitar , but Steve Vai uses a volume pedal.
BTW, there are modern effects made specialy for this trick ... In Boss GT-3 ,there's an effect called "Slow Gear" handle the volume controle for you , so you can focus on playing.

To get the exact tone of Slash or Santana , you'll need a full blown mega tube amp like a Marshall JCM or Mesa dual recto , which will cost you a lot ... Try to tweak your pedal's settings as you can to get similar tone.

To increase sustain using effects , add more distortion (to much distortion can su*k tone and playability) , reverbs and short delays , and of course the amp's volume.

The neck type can of course affect the sustain (neck-thru guitars have more sustain than bolt-on) but it's not as significant as the effect of pickups,amps & effect settings.