Welcome to BYC, Tia!
Sorry to hear about the poopy butt situation. It is certainly a great idea to take a fecal test prior to giving the whole flock a dewormer. May I ask where you found your kit? I've never seen them before.
I had a hen who was prone to poopy butt. Every time it happened to my hen I would do my best to clean her up with warm soapy water (dawn dish soap is what I use). I also trim the feathers too if I cannot clean it all.
Obviously it's a dirty job, so gloves are a good idea. It's easier to have a helper, too. One person to hold the bird, one person to do the dirty work.
I'm not sure what causes poopy butt, but if I were you I would do a search on this forum for "vent gleet". I don't know much about it, but it may be something to look into.
My best to you.