2 Gallon is fine. The undergravel filter is actually kind of rubbish since it's so hard to keep the tank clean.
Technically you don't need one in a two gallon. It won't make a difference.
It's impossible to cycle a 2 gallon. (a process where beneficial bacteria form in the gravel of your tank that help control the harmful chemicals in the water to keep the fish healthy)
Instead of trying to use the undergravel filter or purchasing a whisper filter, I would just recommend making a 75 percent water change daily and a 100 percent water change once a week. No filter needed.
Stay away from plastic plants. If it will rip a pair of pantyhose it will rip a fin in a heart beat, which invites sickness and infection. Try using silk plants instead. They're very pretty and they won't harm your fish.
I also HIGHLY RECOMMEND A HEATER.
Actually, you NEED a heater.
These are tropical fish. They require tropical temperatures. Stable tropical temperatures which only an aquarium heater can provide.
Otherwise your fish will become sluggish and their immune system will become weak. Once again, making them susceptible to illness, infection, and parasites.
You also need water conditioner. Prime is the best, but as long as it gets rid of chloramine and chlorine as well as heavy metal particles in your tap water then you're doing good.
If you need any more information on how to do a water change there are plenty of people on here that would be happy to help you out, me included.