Prime is my preferred product for shrimp. If you overdose, it can bind up a lot of oxygen really quickly. I tend to use an airstone or increased surface agitation if I ever have to use it in a tank where I've got shrimp.
No real negative experiences, though others have had some. (Or maybe it's with other products - because I can't recall specific incidents with Prime)
If your RO/DI filter is working properly, there's no reason to use a dechlorinator. For peace of mind, though? Doesn't hurt anything to add a dose of Prime or Safe to your RO/DI reservoir.
I'm using it all the time. Even in RO water. Dosage for me is: .5ml per 5gal water. Reason: my tap water has 3.5-4 PPM chlorine in it. I have 3 inline carbon filters before RO/DI unit in order to keep reasonable clean water for 6 months. No ill effects with PRIME. Never dose it directly in the tank, thought.