Agreed. If they don't eat regularly they don't poo regularly either. My picky eater goes VERY infrequently, so don't fret.

On the constipated line... the others are correct. Unless you see bloating and swelling at the base of the tail (vent area) then they're fine and just bidding their time.
When I rescued my pastel male, he WAS constipated...badly. I took him to the vet, of course, and they expressed some pretty nasty stuff. But his tail end was swollen to twice his body size! (Not THAT terrible a thing, since his body size was 1/3-1/4 what it should have been). It's not hard to miss...