I don't know why the dechlorinator wouldn't work for all the fish.
What are the active ingredients?
If it just says, 'dechlorinator,' and nothing else added it should be just fine.
In a 5g tank a betta is much more likely to attack other fish. They actually claim quite a bit of territory for themselves. (Not to mention, even with a good filter you would have to change up to 50% of the water once or more each day to keep up with the ammonia output from all the fish/waste.)
If you would like to add additional fish with yours you will need at least a 10g. You should be able to have some tetras with your betta and snail in that size (make sure your tetras aren't the fin nipping kind!)
For platies, if you start with young ones you could start everyone in a 10g, but a few months later you'll need a 15 or 20g, they get about the same size or bigger than the betta and are usually active fish, they need and like their space. Just remember if your getting males and female platies that you need at least 2 or 3 females per male, or he will wear them out from all his 'attentions.'
Good luck!
