Question: I know I can have at least 2 classes, but can I have more? This is of course at the discretion of my GM whether he allows me to have 20 classes if I'd wish. But are there any set rules for this?

Answer: 
This combination would not be any more viable then a regular sized projectile
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From the text on [Shrink Item](http://paizo.com/pathfinderRPG/prd/spells/shrinkItem.html)

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> Objects changed by a shrink item spell can be returned to normal composition and size merely by tossing them onto any solid surface or by a word of command from the original caster.
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This shows that the item will only resize on a command word or by tossing the item onto a solid surface.

If you compare this to the wording of [Reduce Person](http://www.d20pfsrd.com/magic/all-spells/r/reduce-person)

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> Melee and projectile weapons deal less damage. Other magical properties are not affected by this spell. Any reduced item that leaves the reduced creature's possession (including a projectile or thrown weapon) instantly returns to its normal size. This means that thrown weapons deal their normal damage (**projectiles deal damage based on the size of the weapon that fired them**).
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This shows 2 things. First, that any resizing while being fired would be specifically stated in the spell. Secondly, regardless of item size, the damage is based on the weapon which fires it.
Projectile weapons won't work like this RAW, thrown weapons might depending on how your DM handles the unshrinking on impacting a solid surface i.e. your target. It's noteworthy though, that while the command word can only be used by the caster, that caster need not be the thrower and might have readied an action before someone, like a monkey familiar with the Throw Anything feat for instance, threw something that might not be a "conventional" weapon.
On a related note, it may be better not to bring up this specific spell when trying to convince your DM that it would be really cool if your raven familiar could deliver command words for you.