Guppys (Lebistes reticulatus) are tropical fish, growing to a maximum of 2" and do best at temps of around 68F-75F. Goldfish (Carassius auratus) are cold water fish, growing to up to 24" and do best...
Yes they can and do. That is why smart people with fancy guppies do not allow the plainer ones in with the fancies. The fancies will not associate themselves with the reggos.
You will notice that the male guppy has a long pointed anal fin. They use this to transfer sperm to the female guppy via the gonopore (hence the constant chasing). The dark spot on the female is...