Aqadvisor has been an excellent resource for me when trying to determine stock. It can tell you what species can live together and how much filtration you will need.
Guppies are great but finding tank mates for them is harder than a lot of people think. A lot of fish bully them. I have a 55g with guppies, Celebese Rainbowfish, Orchid Endlers, and a pleco (though I don't advise getting a pleco if you want a fish to help you clean. Those guys are waste machines!) they all get along great and make a beautiful group.
Of course your tank is smaller and you want to make sure your species will be able to school. I would absolutely suggest looking into getting some Endlers. They produce a very minimal amount of waste and they are active, colorful little fish.
You can always get non-fish tank mates too- such as ghost shrimp, African dwarf frogs, etc.
Good luck!
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