Whether a fish is shy depends on a lot of factors. Each individual fish has a unique personality, just like humans, and two fish within the same species may behave totally different. After the fish's personality, the most important factor is probably how comfortable they feel in their tank. There are lots of different things you can do to make them feel comfortable, some good suggestions being having lots of décor (floating plants work wonders) and having some dither fish. Tall-bodied tetras will work well.
As far as the fish, the convict idea isn't a bad one but I think C. Nanoluteus would be better. They are more colorful, smaller convicts. You could easily have a breeding pair or maybe a trio in a 23 gallon. That size is large enough for a convict but a pair might be tough on the female in this size. Wet spot tropical fish has nanoluteus for sale as well.