Hi and welcome to the forum
Rams are better off in prs rather than groups so get 2 instead of 3. Alternatively Apistogramma cacatuoides are nice fish and there are numerous other small cichlids like Pelvicachromis subocellatus that are peaceful but they are African so might not fit with your South American theme.
Tetras do best in groups of 10 or more so increase their numbers a bit.
Keep common Bristlenose catfish or Peckoltias, or Otocinculs (as mentioned above).
Corydoras also need to be kept in groups of 10 or more. However, it depends on actual tank dimensions (length x width) as to how many you can keep in the tank. If you do get a bigger group you can mix species and have 5 of one species and 5 of another.
Again tank dimensions will determine if you should keep angelfish. If you have sufficient room then get a group of 6-10 small angelfish and grow them up together. When they mature and pair off, keep one pr and rehome the others.
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If the aquarium is new and does not have an established filter, then either cycle the filters first, of add the silvertip tetras and let them cycle the filters, then add the other fishes after the filters have established