First let me say that I've kept the freshwater clams, and that is very difficult to do. They don't eat nitrates, they are a filter feeder, so garbage and algae in the water are their food source. They also eat A LOT. The longest I was ever able to keep one alive was 2 yrs, and that was such a challenge.
Clams will need a sand substrate because they need to be able to bury down into it and room enough to move around. I have found that if the tank is large enough (55 gal or more), you can use a tub of sand on one end of the tank, but it's difficult to keep clean.
High nitrates will kill clams fast, as will any ammonia, or nitrite, or pH fluctuations. When a clam dies in the tank, it pollutes the water horribly and ammonia levels tend to spike very high, which tends to kill a lot of fish.
I really don't suggest keeping clams in a community tank... if you want to venture into clams, maybe try offering a smaller tank to just them, learn enough about them, and then consider keeping them with fish.
As for Dana's comments, and Blue's, too...
I'd avoid the threadfin rainbows because the danios would likely chew them to bits quickly.
Neon dwarf rainbows are a definite possiblity, and the list you came up with was a pretty good one. Just remember not to choose too many fish... overstocking will cause every problem you can imagine and then some.
The pristella tetras are nice, very peaceful, but get a little larger than most of the others you have mentioned so far. They need peaceful tankmates, and are prone to ick from stress. With the danios in the tank, I'd shy away from the pristellas.
As for plecos, I'd say choose 1 for that size of a tank. Food supply and territory will say a lot, and the smaller species that have been mentioned will each eat a lot.
Now, to approach the differences between corys & plecos:
The easy way to distinguish between the two is their function in the tank... pleco = algae eater and cory = bottom feeder. This will not apply with ALL plecos, as some will eat wood, but the funtion in the tank is different, and it is safe to keep both so long as everyone is fed properly and the tank is not overstocked or crowded. Do not rely on a cory cat to eat algae, they won't. Corys need "meaty" foods, such as black worms, tubifex worms, and various others, and they will also eat flake food if your other fish miss it.
For loaches, that's fine if they don't interest you, there are plenty of other people to love them. You mentioned you didn't see any that interested you, so let me just share my favorite with you, see if that peaks any interest?
botia angelicus http://www.aquariumfish.net/images_0...licus_w240.jpg
With kuhli loaches, be sure the grave is fine grade gravel, as they spend most of their time burried in it. If the gravel is too coarse, it can cause damage to their bodies. Loaches are scaleless, so any species will require really good water quality all the time. They are affected quickly by ammonia, nitrite, and nitrate levels.