Chem,
Here is a very good chart showing the placement of instruments in the orchestra.
http://www.kennedy-center.org/nso/orchestra/images/orchestra.gif you can pan your instruments accordingly. Panning will help enormously as it seperates the sounds spatially.
Also try layering the Sonic Implant ModWheel espr 1st vlns with the VSL Vi-14 p_mf_f 3 layer (also mod wheel) You can create the part on one and then copy it over exactly to the other. You might have to edit the attacks so they're not identical (just move the attacks by a fraction in either VSL or SI so you don't hear identical stops and starts of samples. Use the mod wheel to create swells and expression - you might have to practise. VSL strings are awesome when used realistically and not played like a keyboard part.
Watch volume levels for harshness. Also try a little EQ if something is too shrill or too bassy but you shouldn't need a lot of EQ with these good libraries. Volume is often the culprit. Listen with headphones once you have a general mix to see if something is way too out in front. Also have at least two different sets of speakers to reference mixes. After a mix, play it on a home or car stereo or even a cheap boom box (or all three.) Also play something similar (like a real orchestra) that's top notch sounding and compare the body of sound next to yours. Do you have the same full bass, clear mids, and brightness in the highs or two much in any categorie?
Don't worry, just have fun learning new principles all the time.
Best,
Dave Connor