I personally wouldn't add that many of those cory species to a 20g high. But, that is just my preference. Those species get a little on the large side. I would cap out my cory stock at more like 6 for cories of that size.
Just a note though, I really wouldn't be too concerned about them getting lonely. In my experience, as long as you have a few other cories of any species, they seem to do just fine. I have kept plenty of singles (a single of 1 species in with cories of other species) over the years, and never had any issues at all. They won't shoal up quite as tightly, but they will often hang out and seem to be just as comfortable as with more of their own species IME.
If you do try more, just be sure to keep an eye on your water parameters and make sure to keep up on pwc.
As for the rasbora, I don't know how big of a town you live in, but chains will generally take in just about any fish they can fit in their tanks as an "adoption." I am rather surprised your lfs won't take them. Most that I have dealt will take in stock, just possibly not for credit. You can try craigslist or asking friends with aquariums.
