Forming a compound with the second word starting with a vowel

Question

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).

Answer 1

In Latin, the rule is simply that the connecting -i- does not appear before a vowel, as in your magn-animus, or using a third-declension adjective (like rapax):

grandaevus "aged" < grandi- "great" + aevum "age"

So rapacatrox seems to be correct. The exception is that the -i- does appear when the first word is monosyllabic, e.g.:

triennium "three-year period" < tri- "three" + annus "year"

For Greek, the first of the two vowels is lost, e.g.

αὐτάρκης "self-sufficient" < αὐτο- "self" + ἀρκέω "suffice"

The exception is when the second word began with digamma, e.g.

αὐτοετής "in the same year" < αὐτο- + (ϝ)έτος "year"
φερέοικος "snail" < φερε- "carry" + (ϝ)οῖκος "house"

Though in such cases there is sometimes contraction:

ἀργός (long ᾱ) "idle" < ἀ- "not" + (ϝ)έργος "deed"
Δημῶναξ (personal name) < δῆμο- "people" + (ϝ)άναξ "lord"

When the second word begins with a rough breathing, this aspirates a preceding voiceless stop:

αὐθάδης "stubborn, self-pleasing" < αὐτο- + ἁδ- "please"