adding the store's water isn't as bad as everyone makes it sound. You don't dry off the fish and sterilize them before you add them to the tank, so if there are any pathogens present they will get in either way. Secondly, fish don't get sick just because there are pathogens around, they get sick when they are stressed and then whatever pathogens are around (which there are always some) have a chance to cause illness. The only real issue with the water in the bag is unnecessarily adding ammonia to your tank.
Very few fish need a significant acclimation. Sensitive fish like discus, fancy plecos, cardinal tetras, etc need a drip acclimation. Once this is performed they should go in. IMO
it is more stressful to cram them in a tiny container instead of just letting them in to the nicely setup display where they can join a school or hide if they prefer.