Definitely not pencil fish for that tank in my view, they want to dart in long lines. Wait until you have a longer tank. Sparklers are lovely, but can be feisty, and they like to lurk. They are quite easy to breed. When they do there's some chasing and hiding. I find the habrosus very docile. When young they explored more. They look sweet lying on leaves like they were sofas. They can be shy depending what species are around them. If you ask I think many people may say they wish they'd got a bigger shoal of their main fish, so maybe more axelrodi? Then save the pleasure of adding one more species later, once things settle. The sparklers may like a period being boss. And the habrosus might adjust better to a tank with fewer species. I did wonder if clown killifish might work?