"... in order to avoid two same vocals together."
You're a bit wrong here about the rules to apply in order to avoid cacofonía. Even if avestruz were female, the proper way to write it would be "la avestruz". In order to apply the "cacofonía avoid rule" (sorry for the expresion invention) you need two conditions:
- The word must start with an "a" (or "ha" for the case)
- The accent must be in the "a"
For example, you have to say "el hacha" and "el ábaco", but on the other hand you have to say "la apertura" (the accent is in the "u") and "la ambrosía" (the accent is in the "i").