About the 80's lead sound that involve many guitars at once, I believe you mean harmonized guitar ...It's usualy based on a normal line acompanied by another playing the 3rd, 5th , 8va or whatever interval but in the same scale of course not to sound disonant.
You don't have to get more than one guitar to play harmonized riffs, you should be able to play nice harmonized leads with a decent harmonizer like the one I got in my GT-3 pedalboard ... But if you can afford an Eventide (the real pro harmonizer) , then get it cuz most of the bands you listen to probably use that one ... A new one would cost around $3000 !

For finger tapping technique , I suggest you learn it from Van Halen's stuff first before going more complicated like "Steve Vai" ... You should also learn one-finger tapping before multi-finger tapping .
For one-finger tapping I mainly do it with my middle finger like Vai so that I can still have my pick held as I normaly do between index & thumb, but other people like Van Haln & Paul Gilbert do it with the index ... Anyway , You should learn to tap with all your fingers if can.

The flowing and smoothness of the notes is realy dependant on what's going in your mind more than anything else , you should be able to play any riff both smooth & hard... generaly for tapping it should sound very smooth... Try to relax your mind & focus on the smoothness of the notes instead of focusing on speed & you'll get it.