Those composite 2x4's will sag in the middle, make sure to put in some squash blocks. The fiberon board is a 5/4 board (15/16") not a 2x6 so plan accordingly for code compliant height of railing. Just use trim head screws to attach the top it doesn't look bad and it will hold the cap very well.
Just saw this 2 year-old thread today. Thought I'd give you my two cents on Ipe. My house is at 6600 feet in Colorado Springs. I have 800 sq ft of Ipe decking, built over 10 years ago. The Ipe shows NO signs of wear, including color! I have had a time trying to stain/protect the wood because it is so dense. Even hardwood deck stain does not penetrate this stuff. I finally found a material out of Florida that i applied 2-1/2 years ago. It shows no sign of wear or discoloration. It's called Wood Rx. I've tried everything under the sun (pun intended) previously - Penafin, Cabots, TWP, etc. I applied stain every other year. Nothing compares to Wood Rx. It does leave a very slight orange hue on the wood, but it seems impervious to the sun and weather. Same goes for the Ipe. When I built the deck, Ipe was $1.57 a lineal foot! Let me know if you'd like to see a pic.