Some dechlorinators have slime coat boosting ingredients--Stress Coat, for instance, is a brand that contains additives that are supposed to stimulate slime coat growth. I prefer to use dechlorinators that just do the basic chlorine/chloramine/ammonia neutralizing, without any of those weird additives like aloe vera. After all, how does it make sense to put extract from a random desert plant into a tropical fish tank, lol? Better to go with a more basic product like Seachem Prime.
A lot of people have used the pea method with success, I have no reason to believe it would cause catastrophic internal damage. It's simply a small amount of indigestible material that generally passes through with no complications. You should only give your betta a small amount, about one pellet's worth. Some people feed their bettas peas once a week, but that isn't really something I recommend because they don't get any nutrition from plants--if you're going to add fiber to your betta's diet regularly, it would be better to do so in the form of frozen brine shrimp or daphnia. They have tough exoskeletons that the fish cannot digest, and they pass through their systems as fiber, in much the same way as plant matter.