Hmmm.... Nice choice. While I do agree that craft foam armor can look really amazing (google the LoTR High Elf Armor tute for details) I think in this case I agree, it'll add bulk.
First, I'd wear two layers, a body shaper then the catsuit (actually, I'm way to pudgy to consider this, but if it was me with my 23 year old body, that's what I'd do).
Second, I'd use a technique called Trapunto and actually quilt the catsuit to get the armored effect. You could paint the armor sections with a slick fabric paint after quilting to really finish it nicely. Major advantage to this is it remains fully flexible.
Trapunto is stupid easy, but time consuming.
You sew two layers of fabric together in the shape you want (in this case, anything that stretches in the same directions as your cat suit- you could use an old tee shirt and trim away the unwanted bits).
You very carefully put a hole in the center of the back layer.
You stuff the pocket you've created with as much or as little polyfil as you desire.
Sew up the hole.
I've done Trapunto where most of my pockets were under 1" square, some as small as 1/2" around.
This has nice pictures, but the stitches are huge, I machine sew the sections for strength. : http://www.allpeoplequilt.com/techni...uilting_1.html