Hi mama! Honestly, kinilig ako sa advance planning mo hehe, a lot of us didn't think about this until we were already struggling, so smart move! The 24mm is usually good for smaller nipples, but since you mentioned your left one is inverted, I'd really suggest having a lactation consultant check you once baby arrives kasi sobrang importante ang fit, wrong flange size makes pumping painful and hindi efficient pa. A few of us moms found out during the first week pa lang of nursing kung anu talaga ang size namin, kaya honestly baka mas practical na i-order mo muna ang 24mm, then if it doesn't work perfectly, you can always get flange inserts or size up to the 27mm without breaking the bank. The good news is maraming options, so don't stress, you'll figure out what works for you once you start!