Yes they are very harmful and you should be careful with them. If you use metal pellets they are very harmful.
No, as long you don't use metal BB's, any plastic, 6mm airsoft BB's are good to use.
Because the air pump in a bb gun makes the bb feel like a pinch
Yes if you get hit with one it will hurt
No
Yes
No.
YES!
Hurt, Yes. Kill most likely not.
Yes it could cause problems.
YES! Very much.
70 FPS is considered low for a bb gun, of course if you were shot with it it would still hurt.
Yes it does hurt to get shot by a BB gun, due to the hardness of the plastic pellets, and the speed they travel out of the BB gun. The effect only lasts 2 minutes depending on what type of BB gun was used.