Dont worry sometimes snails can take days to move. The only real test is if you lift them out, do they smell! If so they are dead.
With ref to introducing shrimps. I do the following
1. Get a suitable size bowl, and empty your shrimps from the bag they are in with the water into a bowl. I like to chuck a piece of moss/plant in for them to hide under and not get to stressed.
2. Take a piece of airline hose with a tap on the end(To allow you to adjust the flow speed)
3. Create a siphon and slowly drip water from your setup into the bowl over 45mins to an hour.
4. Then catch the shrimp using a net and put into your tank.
5. Dis-guard the water in the bowl.
This gives them plenty of time to acclimatise, as they are sensitive to temp/condition changes.
Andy