Setting up a 12 gallon Nano Cube

Might be a bit on the heavily stocked side. I'd probably go with the neons, if you can find healthy stock, simpy because the cardinals get a bit larger. I'd probably go with just a betta and 6-7 pygmy cories instead. If you have that many fish, you'll need to be very good about maintenance, and I'd encourage you to have it heavily planted as well.

For substrate--sand is my favorite. Variety of colors, easy to clean, and easy to mix with fluorite/etc for a good, rich plant substrate.

That would allow you to have the neons and the betta--you'll still need to be careful, but it should work. In such a small tank, you won't really be able to appreciate the schooling of the neons--they need a large open tank, but they will do just fine (once more--as long as you find healthy stock).

Looks like you're doing the same thing I just started. I have a 29G tank that I plan on having neons, other tetra family (in the future) , and a betta along with a bottom feeder or algea eater. It's only been up for afew hours now but they seem to be doing ok. The betta lunged at the neons when he first saw them but I'm assuming he's never seen anything other then betta's. Just a tip though. When I picked out my betta I had alot to choose from and they all seemed very healthy. I took the most agressive betta (unfortunately he was my first pick to take home.) and put him next to other bettas on the stand. I was looking for flaring (gils out fins erect) hoping to find a passive one. I rotated through all of them. I did find one that seemed more dosile then the others, he only flared ,if at all, when the other flared first. I started out in this hobby hoping to keep a peacefull community tank with a betta in it and not many "capitive" breaders if any. Neons are ,in the general opinion, very difficult to breed. So they fit my plan. The big down side to neons is they tend to be very touchy to water changes. Something that has always comes up in every thing I read about them. So having a fully cycled tank would be a big plus. I don't at this point in time but I was close to finishing a fishless cycle. Hope all goes well for you.
-Neo Sithlord

I would stick to amano or ghost shrimp. They are the smallest of the shrimp for freshwater tanks. Amano are the only ones that really eat algae. The ghost are more scavengers. Look at getting close to 10 of them if the tank is well planted. Both will eat leftovers on the bottom so you don't really need to wait too long for algae build up.