^ Honestly, i wouldn't add any fish in there personally since you already have Cherry and Tiger shrimp living there as it is...
Give the shrimp some time to breed, and before you know it, you'll have an 8G full of different size Cherry Reds and possibly tigers running around... but on the other hand, if you add any fish in there, chances are they will pick off a lot of the baby shrimp before they mature...
So really depends on what stock you ultimately want to keep in there long term...