I'd try to trap as many of the snails and shrimp as possible before killing them off. The ammonia spike from them decaying, if there are as many as you are saying, would cause issues for the Endlers.
The Endlers should take care of the scuds and baby shrimp.
I could swing by on my way from Philly later in the week and net/trap most of the shrimp and haul them away for you ;D
I used a few pennies in a 2 gallon bucket to rid plants of the snails I couldn't find before transferring them to my tank. Wasn't a couple hour deal, but it worked. I don't see the pipe being an issue. Soak it in hot vinegar to clean it, then give it a cold water rinse and you should be fine.
But if you plan mass genocide, I would seriously consider placing the Endlers in another container and doing water changes daily or every other day. If you have filter media in the container (even with no circulation), every other day should be okay. Just keep it bare bottom and vac the bottom to minimize ammonia spikes. Small cheap plastic containers, paint mixing containers ($1-$2), will work temporarily. I would vacuum the tank as well as you can every couple of days to remove as many bodies and decaying matter as you can until everything stabilizes before putting the Endlers back in.
I will come over with a bucket and remove as many shrimp and snails as possible. I'm heading to Philly tomorrow and coming back sometime next week, my schedule is a little flexible.