1. He stopped smoking.

(Does it mean he stopped smoking forever or he stopped smoking for the time being and he will smoke again later?)

2. They also sell big, soft pretzels.
3. They also sell big soft pretzels.
4. They also sell big and soft pretzels.
(Which one is grammatical?)

5. They also sell soft, big pretzels.
(What about this one? Is it grammatical?)

1. This could mean either one, depending on the context.

#2 is best. #3 is incorrect. #4 is very awkward.

#5 is correct, yes.