Whoa! Dude, you've got a 55 gallon just hanging around? And you are going to start with the 5gallon?
Oh, Welcome aboard!
As much as it may be a great idea, I couldn't do it.
Personally, I'm not fond of live bearers, trying to deal with all the fry if/when they breed... The endlers may be a good choice, others can comment.
Do you know what your water parameters are? Hardness, pH? That will affect what you can choose.
A 5 gallon won't be as forgiving as a larger tank but certainly easier to change out the water.
Plants? Or plant less (I'd recommend plants but some don't like to put them in even though it helps with the ammonia and looks great) if you want plants, what light do you have?
Depending on your water you could go with something like a group of Pygmy Cory , dwarf Rasbora , Mosquito Rasbora . A single species anyway. Those are some of the smallest that will allow for a decent sized group in such a small tank. Click on the highlighted names to go to the profiles... if I spelled them right. You might be able to put 8 in but they all need a group of at least 6.
Add a tiger snail or similar to provide a little cleanup duty. I find four in a 37 gallon do a nice job so don't go for a large one.
You know, if you just bump up to a 10 gallon, it opens up more options for you and a 10 can't be much more than $15... what other equipment do you have that you are going to use for the 5? Heater, filter, light?