When you add shrimp to a tank you don't just let the bag float and pour them in like you do fish. As everyone has said, they are extremely sensitive to parameter changes; especially PH.
Get a plastic or glass bowl and open the bag and gently pour shrimp and water in. I use a glass bowl so I can tell if any are stuck to the sides.
Add 1/4 to 1/2 cup of tank water every 10-15 minutes three times.
I remove my Betta and add shrimp to aquarium. I'm bad and dump the whole bowl into the tank and don't worry about the breeder's water.
I give the shrimp 30 minutes to adjust, float Willie or Si in the tank for 10 minutes and then let them go.
I have 20+ RCS in Si's 5 gallon tank and all are at least 1/2" in size. The breeder I buy from sends a bunch of extras. I will soon be ordering another 20 for Willie's tank.
I have heavily planted tanks with moss and driftwood. I also bought shrimp houses and half-tubes so the shrimp can hide when they molt. Got them from www.plecocaves.com
When I tried adding three-five shrimp at a time, the Bettas had a field day. At the breeder's suggestion, I added 20 and had no problem. Go figure. Guess Si was overwhelmed.
He actually hid from them for 30 minutes!