With your harder water you are in a similar boat to me but we have loads of good options.
Guppies and Mollies will do well in the harder water as live bearers but... I personally prefer Platys and Swordtails. Mollies can be a bit aggressive and chase other fish and Guppies with their flowing tails can limit the choices you make with other fish.
Instead of your Tetras
@Essjay has suggested some great fish above with the small Rainbowfish and Emerald Rasboras but you could also look at Celestial Pearl Danios or Galaxy Rasbora and Rummy Nose Rasboras.
Which species of shrimp have you seen? You could get Cherrys no problem and start with a group of 10 or so and let them breed or you could also get Amano shrimp which don't breed in freshwater and get a bit bigger but I'd get 5-8 of these as you wont really see 2 in a tank this size.
For larger fish, it depends on if you want the smaller fish. So if you went with the Rasbora species or Rainbows we mentioned I'd go for Thick Lipped Gourami - there is a red form that is really beautiful. Or if you wanted to focus on the bigger fish like the livebearers - Swordtails or Mollies - you could look at some small Central American cichlids like Amatitlania Nanoluteus, Amatitlania Cutteri or perhaps some Rainbow Cichlids.
Plecos in this size tank for me can take up too much room and will take up more space than having the benefit - unless you really want them. One option could be a Red Lizard Whiptail, they are a man made hybrid but the original species are from a really wide distribution so I wouldnt be too concerned about the hardness with them.
https://www.seriouslyfish.com/species/rineloricaria-sp/
So as a few examples I could suggest
3 Red Thick Lipped Gourami - 1 male, 2 females (males have longer dorsal fins)
3 Platies - any colour but start with 1 male 2 females
6 Rummy Nose Rasbora
8 Emerald Rasbora
2 Red Lizard Whiptails
or
2 Amatitlani Cutteri or Nanoluteus
3 Swordtails - any colour but start with 1 male 2 females
8 Dwarf Neon Rainbowfish
Also noticed you are from Derbyshire - if you are not too far you should check out Wharf Aquatics in Pinxton, one of the best stores in the UK and well worth a trip. You will also be able to get some of the rarer species I've mentioned there
Wills