Nettie, for flats or prefolds, if you have a good fitting cover, then don't stress, because if some poop sneaks out the flat then the cover can catch it! At the moment we like our Motherease Rikki covers (not the cheapest though) and our Tots Bots size 1 covers. They both have gussets at the legs which allow for a snugger fit. I think Bummis do a cover with gussets too.

I've found BumG one sized pocket to be pretty grandparent/dh-friendly if you pre-stuff them so they're ready to go. Once they're pre-stuffed they're as easy as an AIO or disposable! I've found they fit newies pretty well too!