Hello, hope you are enjoying your fishy friends!
A 5 gal. tank is quite small, but 2 guppies and 2-3 cory's wouldn't scare me at all; however, I wouldn't suggest adding anything else. Also, how old is the tank? Has it gone through it's
nitrogen cycle? You should consider not adding anything else until that cycle has completed itself. Also be sure to test your water parameters often (maybe every other, or every third) day for a month or so to be sure major fluxuations have subsided. Cory's are very peaceful community fish. The only way you could have too many is if the tank is becoming overpopulated or has a smaller bottom surface area. I have personally experienced much success with mixing different kinds together as well. Hope this helps! Happy fish keeping!