Pretty much any type works. My favorites are Fundulopanchax mamfensis basua which is pretty blue with red speckles. If you want to do a species tank with salt, go for any of the Nothobranchius. They're so small and cute and the most vibrant fish I've ever seen. Plus it is fun to collect and hatch eggs. With Fundulopanchax, they'll lay eggs that may hatch and grow in the tank -- I have 2 large fry that have done that and countless tiny fry. So much more fun than guppies.
I'd consider them aggressive because Fundulopanchax will attack each other / other fish -- especially the males. They're super hardy though, just keep one male in your tank.