I'm gonna be tagging you to help me name all my fish, thank you! Those are awesome suggestions.
I think I'm gonna pick Stardust! Works well with Ziggy of course, and works whether s/he turns out to be male or female
I've never kept kuhli's so I can't help you there I'm afraid. Check out Seriously Fish and use their search function for kuhli loaches
This worries me I'm afraid. Guppies can't live in a tank that small, they need some swimming space. There really aren't many fish that can live a long happy life in 20 litres of water...
In your shoes, I would revamp, but in a bigger tank. a 75 L tank will give you so many more options for fish, and be much easier to maintain a stable environment for whatever fish you do get. When something goes wrong in a small tank, it goes wrong fast. Not much water to dilute it, you know? So small tanks need water changing more often. A 20 US gallon/75 litre can maintain more of a balance, you can get more plants and dramatic wood arrangements, and far more options for fish.
Those betta trio type tanks - Do you know the dimensions or make of this tank?
I've never yet seen one that is suitable for bettas. Some only give like a half gallon per fish, or even less! Plus most of them have shared water, which stresses bettas out. They're able to sense the other males close to their territory, yet can't fight and chase them away. Bettas are also called Siamese fighting fish for a reason... and they rarely have any room to swim and explore in a tiny trio tank...
I wonder if your friend's set up has led you to believe that bettas are good in quite small tanks? The thing is, many betta breeders separate male fry into individual jars and things, because otherwise males will fight. But it's only meant to be temporary, while they're raising their fish. They live in pretty large territories in the wild (not in puddles, as so many places will tell you!), so they really should then go to homes who keep them in at least 20 litres/5 gal tanks. Condemning them to live out their lives in a tiny half gallon sectioned tank, with another male either side of them that they can sense through the water... it's just cruel I'm afraid.
What about that large 300 odd litre tank you were talking about setting up?