So I went to two of my LFS today - Aquajardin and Maidenhead. The Aquajardin is very small and didn't have any of the fish mentioned, however Maidenhead is amazing and has a fantastic range of unusual fish.
If you're having trouble finding a scarlet badis or want another suggestion, I would consider a group of sparkling gouramis to go in the top of your tank and maybe some more shrimp to keep yours company. I can't imagine the gouramis would be big enough to eat the shrimp (correct me if I'm wrong), although you would need to lower the pH substantially.
I saw sparkling gouramis in MA - lovely fish. I've written off buying any more shrimp unfortunately though, I've simply not had any luck with them and can't bare the thought of killing any more! However I guess if the problem is about my water, if I went this route and was using RO water it may not be a problem!
Oz should be OK with a betta if you chose that route.
Scarlet badis males are usually pale in shop tanks and don't look very inspiring. If they are the same as other fish, they will colour up when settled in. You could keep those in your tapwater without RO.
If you used RO to lower your hardness and pH Andrea's suggestion of sparkling gouramis would work, a small group of them. They would need an air gap like a betta. If you had a mixed sex group they might even breed. They are tricky to sex so you'd have to buy three or four and hope for at least one male and one female.
I would prefer to stick to my normal tapwater, however if it's just not viable I'm happy to go this route!
I saw in MA 'neon blue badis' - is this the same species but a different colour? I wasn't entirely sure so didn't want to make any decisions until I'd checked here! They were a greyish colour but with a pretty black pattern, and as you mentioned I imagine would colour up to a neon blue as described. I was very tempted by them! So just two - when you say a pair do you mean two male or MF or FF? This was the closest I found to the scarlett badis, though I think there is another shop I could try a little further out.
I spent a lot of time browsing in MA, such lovely fish! I was hoping to get some names to talk about here but my phone had died and I didn't have anything to write down any names. I really like this shop - the shop worker I spoke to really knew his stuff when we got the betta for work and they have a lot of really helpful information booklets around about everything to do with fish... they seem to genuinely want to make sure the fish are being looked after properly once sold!
Another ermm... interesting question! I was wondering if it would be OK to take my future fish purchases home on my scooter!? I can't see how it would be any worse than travelling home by car... I could either put them in an insulated picnic backpack and wear it, or I could put them in my top box or underseat storage. I don't think either get particularly cold or hot