I understand from Lingua Latina per se Illustrata (chap. 6) that the prepositions ā and ab are equivalent, except that ā is used only before words beginning with consonants, while ab can be used before any word, regardless of its initial letter.
Stylistically, in Classical Latin, would it be considered odd or incorrect to exclusively use ab? Or, put another way, are there any Classical authors that completely neglect the use of ā?
I believe that would be considered very odd. Before certain words, ab is almost never used by any author. Consider for example *ab te, which is found 0 times in the Hewlett-Packard repository. If you replace that with a te, that's 831 results, and 275 for abs te. Similarly, ?ab me gives you only 1 result; it happens to be from Cicero, but I suspect it to be an error in transcription.
So it is possible that no author never uses *ab te or ?ab me; but in certain common combinations ab is still extremely rare in almost all authors, and so it would be considered very odd to always use ab.
A few more queries:
Lewis and Short provide some guidance on the limitations of the pre-consonant use of ab:
[ab] has become the principal form and the one most generally used through all periods—and indeed the only one used before all vowels and h; here and there also before some consonants, particularly l, n, r, and s; rarely before c, j, d, t; and almost never before the labials p, b, f, v, or before m.
So the use of ab before consonants can be summarized:
Here is a relevant passage from the second-century (CE) grammarian Velius Longus:
antiquos scimus et abs te dixisse: nos contenti sumus a te dicere. scimus ipsos et ab Lucio dixisse: nos observamus ut [ab] praeponatur his nominibus quae a vocali incipiunt, ut cum dicimus ab Olympo. non adsumitur autem haec b littera, quotiens nomina a consonante incipiunt, ut a Romulo.
"We know that the ancients also said abs te; we are content to say a te. We know that they also said ab Lucio; we observe the rule that ab is used before nouns that begin with a vowel, as when we say ab Olympo. But this letter b is not added whenever nouns begin with a consonant, as a Romulo."
As the other answers show, this rule is not completely true of the Latin of all periods, but Velius Longus at least thought it was true of his own time's usage.
