I use something called silver sand, which is a totally inert sand from my LFS. I have no idea what it might be called in the US, though. What's the problem with pool filter sand?
Is it possible to rehome the julii's? They are a larger cory and won't thrive in a 10 gallon the way the pandas will. Rehoming them would also allow you to have a properly sized school of one species of cory, which would be far better for the cories in the long run.
In terms of plants, make sure that you have fast growing stem plants, as they are what makes an NPT successful. In mine I have:
- ludwigia repens
- willow hygro
And some others that I can't remember. You need a lot - you want to cover approximately 75% of the substrate with them.
Some good mid-low cover plants include java fern, anubias, susswassertang and java moss. I'm also looking at introducing some low-light crypts into mine. :)
All of the plants I have mentioned would be fine in a 5 gallon tank as long as you keep on top of the trimming. :) My main problem in my previous tank was that I got lazy with trimming, and I ended up with more plants than water. So trim, trim, trim!