The shrimp would likely get eaten eventually, but the stocking sounds fine as long as you have a suitable filter and a cycled tank.
I would not trust what the pet store has to say about any fish, to be honest. Most employees are only given enough information to sell you things, not to give you sound advice on how to care for your pets. It is your responsibility to research any animal you consider buying for yourself.
Also, don't confuse peaceful and mellow with sick and lethargic--it can be really hard to gauge a betta's behavior when they are hungry, cold, stressed, and scared--and likely sitting in a cup full of ammonia.