Interesting comments. First my thoughts to water parameters
KH and GH are independent. KH is not a subset of GH or vice versa. To keep soft water fish the KH should be below (!) 6 °dH. You might get a way with a little higher for some more hardy species but not with 20. To my knowledge KH is more important for the fish than GH. Unfortunately I cannot cite any literature. If anyone can provide anything substantial to refuse that (
@Byron you seem to have a different take on that
), I am happy to learn something new.
However, before choosing any fish species you should resolve your water problem. (Btw, there are beautiful hard water fish too.)
If you want to "fill up your tank" it is much better to chose one gregarious species and add a decent number of it. It looks much better and also is for the fish much better than a mix of many different species with too few individuals. This also prevents the problem with the "nippy" species. Nipping only occurs in overcrowded tanks, where those species are not present in sufficient numbers, but as single individuals or super small groups which do not allow for proper intra-species interaction.
Of course a second shoaling species is possible if a species with different behaviour is chosen. Danios and barbs are in fact a very good combination. (Check out the Pethia/Puntius species. There are many interesting and beautiful ones.)
Too bad you did not use sand as substrate. This will rule out nearly all bottom dwellers. But the barbs and the snails will take care of that too.
Shrimps and barbs don't mix too well. Only the larges shrimps with the smallest barbs might be worth a try. I wouldn't.
What kind of temp can you provide? Do you expect hot summers/cold winters?
About the bristlenose pleco: Should be easily possible to house more than one in your tank, if you provide enough cover and separate hiding places (up to 4, better only 2). But be aware that you will be pretty easily overrun with offspring, if you don't stick to single sex only!