Yep, I agree with getting a 6 gallon. May or may not need a bigger heater. What do others think on that?
I agree with
@Essjay on the fact that the flow will be betta (Sorry xD)
Definitely get some gravel in there so there is more surface area for bacteria.
I would personally suggest some live plants, it would help with the balance (floating plats will help a lot, good job on them!) You could get an anubis, or java fern, or something. I have got anubis, it is beautiful!!
Put in a few shrimp if you get live plants maybe, just make sure they are big enough not to get eaten. Adult cherry shrimp will do fine I think.
Also, Nerite snails are lovely! They only breed in brackish water so will not overun your tank.
You might want to get someone to back me up on this but I have a few years experience!!!
I hope you have succes with your fish, photos would be greatly appreciated!!