First of all, thanks for all the input guys - I'm not used to such active forums like this and really appreciate it. Regarding Chalceus/ pike killifish, I'm quite meticulous as to what goes in to the tank and for now they don't match my criterias appearance-wise. I'm no stranger to keeping monster fish and having grow-outs. This time around I don't foresee any tank upgrades in a short horizon so I'm gonna stick to slightly smaller species. Another thing I'm considering is the sense of scale, and even though a larger fish can be kept in my tank it may not be what I want as it'll make the rest of the tank look smaller.
I'm not sure how suitable for your tank they are, but in addition to the banjo cats, I've always liked the farlowella cats. I know there are a few at least that will stay small.
The farlowella looks interesting, can be a nice addition with a single specimen for diversity.
I'll throw in a second for pea puffers. I've got a single male in a 10 gallon high tech planted. It's him, an Otocinclus, and whatever snails manage to escape his voracious appetite. Pond snails, ramshorn, bladder snails, they all get destroyed very quickly. The only snails that have any longevity in the tank are Malaysian trumpet snails since they spend a lot of time in the substrate.
My male puffer has shared his tank with a large amano and large nerites, both were left alone. Amano died after a large water change, nerites got pulled because they poop too much. Occasionally, I'll see him chase the Oto around the tank, but he's never nipped at him and the Oto is in great health. For the most part, they leave each other alone. In a 20 gallon, you could do several pea puffers, but keeping snails stocked will be a challenge.
They're darn cute those midget puffers! I was looking at them a few years ago as well, for a nano tank. They are on the shortlist for this upcoming community tank.
Many thanks for that link to MFK, it's a practical goldmine with plenty of reading for me!
Small (under 5" TL), predatory and pike-like would include the genera Aplocheilus and Epiplatys killifish. One of the most personable is Aplocheilus lineatus, which is a big & beautiful species. Anything small enough to fit in their capacious mouth will disappear. In an emergent aqua-terrarium situation, they will jump to ingest crickets. They will also take chunks of prepared dry and gel foods and are incessant beggars.
A trio or quad would be suitable for your tank dimensions - a FULL cover is required as the fish are notable jumpers.
I never explored the killifish before, but you're making some good points as to why I should - still I'm hesitant around jumping fish as I plan for parts of my scape to break the water surface. I could get lids cut-out to fit, but I'll face light losses amongst other cons. Still, for the right fish I may do it.