Your yogurt cups should be a nice size to pot up to, David. Start saving those puppies!
Petunias particularly take quite a while to get big. You might get away with leaving them in the 72-cell. I've never started Impatiens from seed so no idea, and alyssum never really get to be huge plants anyway. I'd play it by ear on the sizes and only pot up those that look like they're getting too big for the cell pack.
