4 or 5 Rainbowfish as a centerpiecewill be beautiful! They are very very colourful. have a look at the different types: celebes, praecox, bosemani, turquoise just to name a few.
To put with the rainbowfish you could have two schools of different types of tetra, congo (these are the most colourful i think), neons/cardinal (second most colour), black neon or rummynose.
Also a group of corydorus catfish will just about complete it
There is a profile of the bosemani rianbowfish and the congo tetra in the AQuairum Advice profiles