6 corys and 4 loaches is an aweful lot of bottom dwellers for a 20H tank, which has a relatively small "footprint" of substrate. I would go with either the corys or loaches, not both. And if you go for the corys, a school of 6 of the same kind is more ideal than mixing & matching. In my experience in the past, some of the corys would school with other species, while others wouldn't and would remain by themselves. And a cory by itself (or only with 1 or 2 others of its kind) is not nearly as happy or healthy, and tends to be far less active.
As for the killifish (the black pearl) you say you like so much, I would do some careful research on them. They seem to be a cooler-water fish than most other killies (and most other tropical fish in general); one site says to keep them in water no more than 20°C (68°F). The point is, you might want to investigate this in detail and see whether you can get a tank temperature that will be cool enough for that while being warm enough for the rest of the fish you say you want. Also keep in mind that is an "annual" killie, which means its full lifespan is likely only about a year or so--if you like the fish that much, you may wish to consider getting a male & female and breeding successive generations of them. Just a thought. (Another site I saw said if the water is too warm, their entire lifespan can be as short as 7-8 months. Considering the ones you are seeing in the petstore are likely already a few months old, at least, that doesn't leave much lifespan left.)
The key to stocking a "high" tank (like your 20H) well is to get a combination of bottom fish, top fish, and midwater fish. What you might want to do at first is get your black pearl(s) and then either your corys or loaches. Observe the black pearl behavior and see whether it is mostly a surface dweller or midwater dweller. Then, once you have a feeling for that, get something that will complement that by spending most of its time in the other zone.
Please keep us posted on how things progress.
