For both Latin and Greek, what rules govern the formation of a compound of two words, with the second word starting with a vowel?
I'm specifically most interested in the rules for Latin, since this question was born out of my attempts to form a specific epithet for a made-up species, via a compound of rapax and atrox in that order (the other way around is obvious: atrocirapax); the first thing that I came up with was rapaciatrox using the Latin interfix -i-, but then I remembered that in the case of magnanimus (the etymological root of English "magnanimous"), the interfix was dropped completely. Yet rapacatrox doesn't seem right to me.
That being said, I am also interested in how it works for Greek, since some Greek words that technically start with a vowel are actually pronounced with an "h" sound preceding the vowel (e.g. ἁρπάζω harpázō).