They count if in great numbers or in a heavily stocked tank, but with a betta and 6 neons in a 10, say, 2 shrimp won't matter much.
May I advise against getting GBRs in a 12g? They'll get 3" long, and arn't the kind of fish to stay in a corner and love it, so the general rule is a 20g minimum. You might be okay with 1, but they're also fairly large fish, and can't be counted in the "one-inch-per-gallon rule", which I, personally, hate for its misleadingness and gathered followers, who swear by it.
May I advise against getting GBRs in a 12g? They'll get 3" long, and arn't the kind of fish to stay in a corner and love it, so the general rule is a 20g minimum. You might be okay with 1, but they're also fairly large fish, and can't be counted in the "one-inch-per-gallon rule", which I, personally, hate for its misleadingness and gathered followers, who swear by it.