Any writers on here? (stuck on a point of style)

Trying to write a novel, and decided to change from putting numbers in words (ie ” She was Twenty-six”) to a more simple “She was 26.” Looks much neater in most cases, except when I come to a “million.” For example, one character has to say something about “Millions & Millions of them” but if I stick to my new rule “1,000,000s and 1,000,000s of them” looks very clumsy. Do I need to be consistent with one style, or can I mix them? Not the biggest problem facing the World I know, but I’d appreciate any thoughts. Ta.