"I do not have one brother."
Translation:Chan eil aon bhràthair agam.
Historically it is specific words that cause lenition. The feminine form of the definite article is one of these, thus giving the false impression that words lenite because they are feminine. They are actually leniting because they follow the feminise definite article. Aon and dà happen to be other words and it is nothing to do with what they mean or whether the word after has to be singular or plural.
Since I see you are also studying French and German, words that lenite originally ended in a vowel and you can see this in other languages. For example, the translation of die, Spanish la causes lenition, but the translation of der, Spanish el does not. It doesn't always work, so don't try this in French, although you will be aware that many French words end in e in their feminine form.
No. You use aig before any noun.
Perhaps you are thinking of ag used before a verbal noun, as in ag iarraidh which becomes a' before a consonant - a' dol. This is sensible, since aig and ag were originally the same word. But they aren't any more. Ag changes but aig doesn't. It's just the way it is.