#1
When I read a slide on a tab do I pick or just slide or both?

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12/15

Talking specifically for solos.
#2
pick the string with your finger on fret 12, then with the string still vibrating, slide up to fret 15
#3
pick the note, if the first number is in (x) than you don't have to pick.
鋼の錬金術師
#4
You only pick the first note, then slide to the second one. That's why it's a slide - otherwise it'd just be two successive notes.