I have a 10 g setup with a few species - Microdevario nana, Celestial Pearl Danios and Rosy loaches (Yunnanilus sp.).
The loaches are a lot of fun to watch. They are very small (1.5") - and inhabit the bottom primarily but jet all around and interact with each other. I wouldn't do them in much less than a 10g - as they like to be in a small group - but for 8-10g I think they are a great candidate. I also really like the CPDs - but in the context of the whole tank. If it were just them, I probably wouldn't enjoy them as much as they are often in the background amongst the plants. The M. nana are great schoolers with subtle iridescence.