You can't wait until check-in unless you are going to list all three of your additional adults then! As you probably know, you will be asked everyone's birthdates when you make your room reservations.
$75 extra is going to be a big hit on your room rate. We have one extra "adult" and even the $25 adds up. Our favorite resort is the YC. Last year, the best rate I could find (AP) was $215, so $240 plus tax. Two rooms at All Stars was $118 plus tax and two rooms at a moderate was $168 plus tax. We broke down and did the two rooms, partly at All Stars and partly at Coronado. Loved having the extra beds and space, but we sure missed the location, pool and hot tubs (while we were at All Stars) of the YC. Our conclusion was that we would rather be crowded into one room at the YC. If prices are similiar next time we go, we'll probably split our stay between two moderate rooms and one YC room.