It's more about not shocking the fish with a temperature change. Make sure you check the temp of the water, and if anything have it 1-3 degrees warmer than the tank is. Fish can handle an increase in temp much better than a decrease.
Yeah, as long as you make the water the right temp, you will be fine within 1-2 degrees up or down. If it's cooler, then it's similar to when the rain falls and the water is cooler than their ambient water.
For a 50g tank I would add a whole capfull every 24 hours (I'm at work so can't check, but I'm pretty sure that is the recommended dose for 50g). Remembering that while cycling with fish the Prime is as much to try and protect the fish from Amm, Nitrites and Nitrates as it is for dechlorinating WC water.
Seachem say you can use up to 5x the recommended dose of Prime in an emergency without any harm to the fish.