Sorry that I missed that first reply, DUH! Hahaha
Coroplast is pretty much just plastic cardboard. http://en.wikipedia.org/wiki/Coroplast
It's really cheap, easy to cut, and waterproof.
I use it for making lids and such but it works for backgrounds too.
That's good that you're being picky about your Betta - make sure what you get is what you want! I am no longer that picky really as I have had so many and I know I will have many more, but I'm still rather selective haha.
As for adding the Cardinals and such, I would say get them both at once, here is why; Most people would say add the Cardinals first wait a week or so and add the Betta after. However, because you're going for the Betta as your "main fish" as I've always done (because let's face it they're friggin awesome) I would advise you to get them at the same time; this way you'll be able to return the tetras (at least for store credit) if it doesn't work out. Otherwise you'd have to set up another tank or think of some other solution if the Betta wasn't liking it.
So yeah I would say buy them at the same time, put the cardinals in first (as well as their water) and leave them in for about 15 mins with the Betta in the bag floating in the tank. Make sure you open the bag for the Betta, and turn the lights off.
Edited to Add:
As far as shoal size goes, I would say that you could have 8, but honestly I would cut the Ottos though.
AqAdvisor says with 6 Cardinals and a Betta you're 97% stocked.