You are correct in assuming that the first part of a binomial (two-part name) is the Genus and the second is the species. So...for the guppy....Poecilia reticulata
, the Genus is Poecilia
and the species is reticulata
. Nice to see that you got the capitalization right too! The generic name (Genus) is ALWAYS capitalized and the specific (species) name is NEVER capitalized, even if it is a proper noun.
With that out of the way.....in GENERAL....member of different genera don't successfully interbreed, though this is not ALWAYS the case! It is not, however, particularly uncommon for different species of certain types of fish to interbreed and if you drop down to the subspecies level it is even more common. Most of the time (but not always) the offspring of interbreeding are sterile.
Keep in mind that the classification of organisms is an artificial system based on certain relatively constant and observable characteristics. Living things don't always understand that they're supposed to follow the rules set by humans!!
The results of interbreeding are usually called 'hybrids'. Endgame is right...hybrids do not have special scientific names. They are generally named according to the cross that created them. For example, a mule is a cross between a horse (Equus caballus
) and a donkey (Equus asinus
). Note that this cross occurs at the species level. The mule is then 'scientifically' named Equus caballus
. If the two species of Rams you mentioned were to interbreed then you would have a hybrid fish named Microgeophagus altispinosa