Hello Romucci and welcome to these forums. You'll find many helpful people here and a lot of useful information. Make sure that you:
get the excellent User Guide by Nicholas too:
http://www.cockos.com/reaper/userguide.php
and check out Kenny Gioia's great video series:
http://forum.cockos.com/showthread.php?t=174039
I'm not sure that I understand fully what you want to do.
First off: which library are you using? The Kontakt 5 Orchestra? Or ... ?
Selecting an articulation is often done by key-switches; that is a range of notes outside the playable range each of which selects a different articulation. To select a different articulation just 'play' the appropriate key-switch note then the notes that you want to hear. (Or edit the notes and key-switches into a MIDI item.) You do not need a separate track for each articulation for a particular instrument; you can do all the articulations on one track.
The violin could be set to receive notes (including key-switch notes) on MIDI channel 01; cello on MIDI channel 02 etc.
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If that's not what you want to do, please let us have some more details.