That helped solve my syntax errors but I noticed a few other problems as well.

1) The objects don't seem to be emitting in the right proportions after checking the numeric object values.

2) the instancer is ignoring objects 2 and 4. This is not an error with the object geometry because if I move the objects levels up and down in the instancer (i.e. my sphere becomes object 2 and my cube is now object 3) the objects do appear - I can see the cube that was originally object 2.

Here's my updated creation expression:
int $i = rand(0,101);
if($i > 50) nParticleShape1.indexpp = 0;
if($i >= 40 && $i < 50) nParticleShape1.indexpp = 1;
if($i >= 30 && $i < 40) nParticleShape1.indexpp = 2;
if($i >= 20 && $i < 30) nParticleShape1.indexpp = 3;
if($i >= 10 && $i < 20) nParticleShape1.indexpp = 4;
if($i >= 8 && $i < 10) nParticleShape1.indexpp = 5;
if($i < 8) nParticleShape1.indexpp = 6;

At this point I've started to just create separate emitters to get the desired effect.