That light cost you most? I don't know if I can offer any high-tech-ish advise. All my tanks are just dirt + sand/rocks, and the cheapest light I could find :/
If you haven't gotten a filter yet I'd still do a HOB, perhaps AC20/30/50. They're only a few dollars apart. I don't think 50 on low flow would create a tornado in a 5g, if so you could always stuff it some more. But I like to overfilter.
Fauna I would stay away from schooling fish, they really need a 10g to have adequate swimming space. But you could get away with a couple CPDs, or a betta. I have 11 pygmaeus corydoras in mine
but thinking of relocating them soon. I'd also say 3 platies but they would probably turn into 30 in a month.