Hello!
I think you have a good idea of what you want to do. Right now I am in somewhat of a similar situation as I have a 10 gallon which I am having a hard time deciding what I want to stock it with!
As far as your setup, I believe that you could do more than 5 nano fish. Maybe start with that number and then add some more over time. I would say like 10 would be fine in there if the tank has been properly cycled and has a lot of biological bacteria.
I'm glad to hear that you want to do plants in the tank. If I were you I would probably get a good substrate like eco complete or flourite black sand and then go from there. You could then do anything from like an iwagumi with rocks and foreground plants such as glosso, dwarf hairgrass, dwarf baby tears, etc. Or you could just make it planted and have a variety of plants in the tank. I think the iwagumi might be cool, because due to the short height of the tank, stem plants may grow pretty fast and not get very tall before you have to cut them. I bet it could still be done in there, but maybe and iwagumi would be a better bet, if you have the right setup for it.
Going back to fish, you could do some smaller fish such as cpds (celestial pearl danios) also known as galaxy rasboras, chili rasboras, or ember tetras. Any of those would probably work well together. If you have tiny fish like that, you may also be able to introduce some shrimp into the tank to help keep it clean and interesting! I have not tried the combo together though so use caution! Also I think there might be smaller gouramis like chocolates or sparkling gouramis. I just read about sparkling gouramis and it seems like people reccomend a 10 gallon, however you could probably do a pair in there with a smaller school like 5 or 6 of the micro fish mentioned above, as long as you keep track of your water and make sure that you do regular water changes and that your system can handle it.
I hope that I helped, and if you have any other questions feel free to ask and I can try to answer them
Have a great day and good luck with the tank. Hopefully you can post some pictures or maybe do a journal about the tank so we can see what you decide!