I think I've mentioned to a few people I'm planning a tank like this so have a bit of a mental list
I'm guessing from what I've seen of other London members you have hard water (probably nasty nitrate levels too) which is the same as me (though different part of the UK).
I've tried to work out a Central American biotope in the past but its quite hard as not many other central american fish are imported ever, they dont have much diversity in terms of small to medium fish a few species of tetra but again hard to find so time to look at other parts of the world with similar water.
Rainbow fish are a good candidate, big tough and fast. Loads of colour choices and a couple of different shapes and sizes around. Bosmani, Turquoise and Reds are amazing fish and pretty common really but there are rarer species like Parva, Kamaka, Picta and more that are equally stunning. Just take a bit of time to mature and never look their best in the shops.
Synodontis catfish from the Rift Lakes are good candidates too, I like the white and black ones but there are some great patterned ones. Golden Wonder Killifish could be a good choice, get a decent size and again loads of colour, surface dweller too so out of the way of the cichlids.
Livebearers are worth a shout too - I used to turn my nose up because of how common they are but their colours these days are amazing. I had a group of Mexican Mollies in my cichlid tank and the male was stunning but even the line bred coloured Platys and Swordtails look amazing now I love the Kohaku Swords and Red Wagtail Platys.
There are some other species too that would work - lots of barbs would fit the bill but I'd probably pick a school of rainbows over them as they are more colourful.
Wills