Ross, you aren't going to like the answer. Although most Latin endings can
be changed simply and almost automatically, the rules of nomenclature do
indeed follow every last jot and tittle of Latin grammar. It's a
nitpicker's paradise.
 
The most frequent source of trouble is determining what gender (masculine,
feminine, or neuter) the generic name has. If the name ends in -us, then it
is almost always masculine. (Exception: Venus is feminine.) If it ends in
-a, then it is almost always feminine. If -um, then neuter. But a lot of
generic names derive from Greek or other languages, and they have different
endings. The easiest course of action is to look for the gender of the
specific names that people have used connected with this generic name, but
it's a hazardous method, since people make mistakes.
 
Also, many Latin nouns and adjectives don't end in -us, -a, or -um. But a
Latin dictionary can clear up these words in a jiffy. If the words really
are Latin. And if you have a big enough dictionary.
 
There are some good books out there on Latin, on botanical Latin (the
language or dialect that Linnaeus and other taxonomists actually use), on
the practice of taxonomy, and on the formation of scientific words in
general. Some are technical, others popular. If you're really interested,
I'll put a short list of them together overnight.
 
Andrew K. Rindsberg
Geological Survey of Alabama