Hello all. I am currently a newbie when it comes to fish and aquariums. I had a few beta fish in the 90's

Boy have things changed.
Anyway my son currently has a beta in a 3 gallon aquarium (which is too small I know). We didn't know before we bought it and getting hubby to replace it after all of the money spent on hermit crabs and fish. He seems happy and we monitor his water and have a good pump and filter in there.
I wanted one for the kitchen downstairs but that seems to have turned into a 15 gallon column aquarium. We are currently cycling it. If I could return it I would. Too deep for a beta, doesn't seem long enough for neon tetras (that is what is in there now), and I am afraid guppies (while very pretty) would have little fishes is that true?
Anyway I am looking for where to post and learn and for any advice as to what to put in this darn tank. It is 13.5 x 13.5 x 24" high.
I hope to find some info here any advice is appreciated.
Thanks!
Jana