HI missed this thread looks like you are in an interesting place

Along with your Bosemani Rainbows you could look at some of the smaller rainbows like Kamaka or Dwarf Neon Lazer - bit rarer than something like Dwarf Neon but better quality - Dwarf Neons are nice but are a bit over bred at the moment so prone to disease. The other option would be Forktail Rainbows or an other option for a schooling fish would be Ricefish or Medeka.
As youve mentioned livebearers are a good option for hardwater and really under rated in tanks like this IMO some beautiful strains of Swordtails and Platies - I really like Kohaku and Coral Reds of both species. Golden Wonder Killifish are a good hard water species, quite interesting shape and a surface dweller and they are really brightly coloured.
Instead of your cories you could look at Synodontis catfish from Lake Tanganyika like S.Polli, S.Lucipinis or S.Petricola keep them in a group of 5 and they all grow between 3 and 5 inches - 5 inches is rare though.
The Chromide could be an option but I prefer Central American Cichlids, in a tank this size something like a Sajica or an Ellioti could do well. Rainbow Cichlids are a really nice fish too. Most of the smaller Amatitlania, Cryptoheros or Thoricthys species will do well in this sized tank - but I would knock off a few of the smaller species I mentioned from the list.
A lot of people view hard water as a negative and I did for a long time too (I have hardwater) but there are loads of great options for us to keep. Even at the nano end of the scale Dwarf Puffers do great, Emerald and Galaxy Rasboras, Rosy Loaches, Endlers etc plenty of great options! And if you have really hard water and a big tank you can always go down the rift lake route with Mbuna or Peacocks - though not with the other fish I've mentioned.
Wills