I have a simmons black pocket coil that I have had for about a year.  We decided to go with the plush top instead of a pillow because we figured the pillow top would degrade fast.  I have found though that I need more softness as my shoulder hurts in the morning due to being a side sleeper.  Other then that I love the bed.  My question is what kind of topper would anyone reccomend and what thickness is reasonable to make a difference.

Sometimes even 1/2" can make a difference.

I would recommend a 1" super low ILD latex layer. Memory foam may work for you but for me, even 1" of it causes my lower back to hurt. If you do go with memory foam, go with a higher density layer such as a 5lb or higher density.

For latex I forget who it is - someone here can tell you - but some online site sells something like 18ILD latex. You might even want to go 2" with latex but I wouldn't go 2" with memory foam.