For chat system you don't. However you must have answered correctly the randomly generated question the ninja asks you once first.

I have not tested the sweep function but i doubt you would loose anything at all. You need to clear it once before sweeping so you have proven you can beat it.

For sweep,its fixed reward,like manual i get 195k coins for cat,sweep give me 180k,for foods,manual give me food and materials,sweep give me only food

That's odd. The cat has a fixed amount of coins attached to the health it has. You shouldn't be able to get more or less on that one. Also i'm surprised you dont get materials when you sweep exp. Will try that tomorrow

If you use Ninjas that cause damage to your team you used to be able to boost the coin rewards past the max. From what i remember it counts all damage you do, even to your own team ie Hidan, Kabuto, etc

I can't sweep coin yet. But I did sweep exp yesterday (manual one sweep one)

It did drop material, thou, I'm not sure if it's just my luck, it was 8 of the same one.

Would have to sweep more to be able to tell if the material have any variety. If it does, even if it's always 8 of the same one per sweep, it would be fine.