Hi jeeplassie,  to join the forum!
So you have a 5 gallon tank. It will be perfect for a single betta.
However, if you try to get Corydoras catfish, which is a school fish, the 5 gallon tank isn't big enough for them. You will need minimal 5~6 of corydoras for a school in order to have them behave normally instead of hide all the time. 5~6 of corydoras might be too much bioload for your 5 gallon.
Endlers might work fine if they are the only fish in your tank.
I have also noticed a problem. You have mentioned that your tank is currently receiving sunlight. It is a big no no.  Not only sunlight will cause algae boom in your tank, but also it might overheat your tank water temperature which is very bad for your fish.
This is an article on the importance of the aquarium location.
http://petskeepersguide.com/the-location...-aquarium/
As for the plants, a 5 gallon is quite a limitation. Since it is not high enough for many tall plants. Keep in mind that you will need minimal 2 inch of substrate for the plants to root properly. You also can't fill the water to the very top, so the actual depth of the water in a 5 gallon would be less than ideal for most plants. A few plants come to my mind right away that can fit into a 5 gallon nicely are
Anubia Nana
Java Fern
Java Moss (if you keep it in small portion)
May I ask which video you referred to? Anubia Nana is presence in one of the aquarium videos in the sticky.
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