'And' before a consonant has been (mostly) a throughout the various Elvish languages, however Tolkien changed his mind several times as to what it should be before a vowel.
In the earlier texts, it was ar: ar Meril bess dîn, ar Elanor, Meril, Glorfinniel, ar Eirien (The King's Letter)
However in the later Finrod and Andreth, it becomes ah: Athrabeth Finrod ah Andreth. The point to note here however, is that although one was written later by Tolkien than the other, as far as the language is concerned I believe it should be viewed the other way around - Finrod and Andreth were in the First Age, and thus it would have been written in Doriathrin Sindarin, whereas The King's Letter etc. would have been written in Exilic Sindarin (Sindarin changed when the Noldor came back from Valinor and had to adopt it as their language when Thingol declared that Quenyan should not be spoken).
So where does adh come into it? PE17 lists the changes Tolkien went through when deciding which word stem, or root, a should come from. I won't bore you with repeating them all here, but the conclusion seems to be that he finally decided that Old Sindarin had ath, and Sindarin has a/adh. Adh is my preferred usage of 'and' before a vowel (unless I'm translating something intended to be Doriathrin of course!) although in all honesty you can use whichever form (ar, ah or adh) you feel most comfortable with.