Great list to start off with! Only main suggestion is that you need to add more of each of species. But with tank size limitation, it will probably either be discus or schooling fish.
Discus are jaw-droppingly beautiful, but smaller schooling fish make the tank look much larger than it is. Discus exhibit much more interesting behaviour than most schooling fish, but are also more demanding (need high quality meaty food, pristine water conditions).
It depends on how many fish you want in your tank though. More fish = more waste = the tank gets dirtier quicker.
The rummynose, cardinal and harlequin will do much better in larger schools (10+ each). In the wild, they school in the thousands. So I guess emulating their natural setting by adding more of their own kind together is a good step to happy schooling fish.
They become much bolder and feel safer the larger the school they are in, and it will look much more visually stunning. See:
http://youtube.com/watch?v=gWj71L7khPE
As well, the difference between 6 and 12 fish, then say 12 and 18 fish, is not that noticeable. However, 3 large mid-dwelling schools might cause insufficient swimming space, and it will end up with awkward schooling movement (kind of like a traffic jam on a 3-way intersection). Maybe 2 large schools and 1 smaller school, or just 2 schools together. Up to you.
Another note that's quite important is the fact that large discus can and will probably eat smaller fish like cardinals if they are hungry enough and the prey can fit in their mouths (even if barely). I know a person who kept 4-5 discus (about 3-5" discs) with about a dozen neon tetras together. I asked him if he knew discus often eat smaller fish like neon, and he told me as long as he kept the discus well fed, the neons were fine.
Guess what happens? Next weekend, he tells me he forgot to feed for a day, and half his neons disappeared that next morning.
Cardinals look mighty similar in size and appearance to neons. ;P
As for getting food evenly distributed, I learned a couple of witty ways from members here as well as on my own. I have a 55g and originally had problems balancing food between tetras, gouramies, loaches and cories.
Some ways to outsmart the fishes:
- Lure the top/mid dwelling hungry fish to one of the tank. Feed floating/slowly sinking foods like flakes, floating pellets, etc. On the other side of tank drop your sinking pellets so it has time to get to the bottom. (Pre-soak to sink faster).
- Manually put the food on the bottom (I don't do this as I don't feel like getting my arms all wet everyday).
- If you have more bottom dwelling fish than mid/top dwelling fish, dump all the food in at once. The top fish can only fit so much in their mouths at a time. After a few tries enough food will get to the bottom. Be careful of overfeeding though.
- Mix the food with some water, use a syringe to inject the food to wherever you want. Works pretty well when I used to do this, but I got lazy and stopped.
- Dump the food near fast water current that's pointed downwards (near filter outlet, powerhead).
What I do nowadays is crush the flake food into very fine bits by rolling them between my fingers, dropped near the powerhead that creates a lot of water current at the surface. The flakes go everywhere and the top/mid-dwelling fish go crazy, chasing the bits around. It takes them longer to get the food. Some food gets to the bottom. Then right after, drop the sinking food (shrimp pellets, algae wagers) without crushing them. This way the top/mid dwelling fish can only peck at it and can't grab ahold of it. By the time the sinking food gets to the bottom, they return near the surface to finish off the flake food.