Nothing in this article or in any apprentice agreement approved under this article shall invalidate any apprenticeship provision in any collective agreement between employers and employees establishing higher apprenticeship standards regarding ratios of apprentices to journeymen, probationary periods, or length of the program. None of the terms or provisions of this article shall apply to any person, firm, corporation, or craft unless, until, and only so long as such person, firm, corporation, or craft voluntarily elects that the terms and provisions of this article shall apply.