A 12G Edge is a beautiful tank. I would love to have one!
One problem is it is more narrow than wide. A consideration of what to put in the tank for fish is also a consideration on how the fish or creature exist and their natural habits.
DG to me, seem to hang out a lot more so than swimming across the tank, so maybe it wouldn't be a big deal for them. Cardinals are schooling swimmers which swim horizontally and they tend to get a couple inches long. So although they are beautiful, they would not be the best choice for the 12G Edge. They would be better in a 36G with the Ram and Gourami, really a nice combination (if you eventually want to upgrade I like the bowfront and the 46G is 36" wide, big but doesn't take up too much room
) You could also do a few more fish with them!!!
Guppies are high polluters of the water. Just so you would be aware of what you are getting yourself into and how fast a smaller tank's water quality can come crashing down on you, there is much greater chance of fish death and illness with the smaller tank just in missing one or two pwc!
The CPD like to dart around but they may be a better choice for you. 6 CPD as they like being in a group. They like plants and driftwood, a natural habitat type tank, though I would make some tall grass like plants in a couple places and set it up so they can still swim all around maybe like a figure 8 so they have the most swimming area available with still having the seclusion with some taller plants.
You could also consider a bunch of dwarf Cherry Red Shrimp (CRS) with them. Although they will eat some of the baby shrimp, if you have a large enough bunch they should keep up the population with lots of plants and some Cholla wood and moss maybe a good idea. A couple of Ramshorn snails or a nerite.
You want to be able to enjoy your fish and you will enjoy them most when they are happy too and in the best conditions they will be the most colorful and healthy. Good for everyone and the fish!
