It reminds me gotcha, whatcha and so on. You definitely can say "первого", and people do understand you. But "первава" is a way easier to say. This way is commnoly used and accepted as a normal way. "Г" is in the ending here, and there is a vowel after it so it is "в".

Also there are words like "Бог". I am really not sure how to read "Г" in this case =) There are variations.
Пирог, сапог and so on. Since "Г" is not in the ending here it is "К". Пироги - it is "Г" since vowel is after, but "Г" is not in ending.