Do you know if your 7 platy are all male or all female? I would assume one or the other, or you would have fry appearing, and this is important because they will survive and greater affect the tank's biology.
Assuming the platy are all one sex, then a few other fish are OK. Some substrate fish, someone mentioned corys, and perhaps a Whiptail catfish for something unusual. Presumably your water is medium hard and the pH is basic (above 7) to suit livebearers, so this has to be considered when selecting new fish.
If getting guppy, make sure they are all male, same issue as above. Other fish options with what you have, and assuming medium hard basic water, might be Pristella Tetra, Zebra Danio, etc. These fish need small groups, 6-7 of each. Not suggesting all these will fit, just one species in a group of 6 or 7.
You will note that the fish names above shaded; that means the species is in our fish profiles, and you can click the name to see the profile with information, specific needs, and photos.