They can live together depending on the tank size & the sexes as guppies can interbreed with Mollies so if you keep those together it's a good idea to just have males to prevent possible interbreeding.
In a 5 gallon I wouldn't add any more fish it's a bit small for your guppies too, if you can I'd suggest you upgrade to at least a 10 gallon.
You still couldn't have goldfish or Mollies but a platy with a few guppies would be ok in a 10.
Agree with Coradee, mollies, platies, guppies, or goldfish won't work in a 5 gallon, sorry. For a five gallon, your options are very limited. You could do a betta and some shrimp or snails if you wanted.