Hrm. Normally that website gives better advice than that. It's a sad lonely platy that lives by itself in a 5 gal.
I use AquaOne or API liquid test kits. I didn't like Nutrafin when I used them. The nitrite reading is part of the cycle - as you are doing it with a fish in the tank, you are likely to have a nitrite reading for a while. I would try upping water changes to reduce it.
There really aren't any other fish you should have in a 5 gallon, for three reasons:
- unless it is a very long, shallow tank, like the Petco Bookshelf, its footprint is too small to allow an active fish room for activity
- unless you have plenty of plants to support your filter, the tank will not cope well with the bioload, especially if you are still getting nitrite readings
- a betta is unlikely to enjoy sharing such limited space with another fish
Basically, if you want more fish, you need a bigger tank.