rubber balls are not the greatest, my ducks show no interest in them... ether that or they are scared of them. The jingly parrot balls are better because they can easilier make the noise.
None of my ducks have ever been able to break any of the toys we've gotten them... and they have a LOT of toys. I do take a look at the toys every now and then just for good measure... that and to clean them... sometimes they knock them onto the floor of their pen and poop on them. lol.
nibbling seems to be okay, but if they strike you, or peck you hard... you can tap their beaks and tell them "No Biting!" in a firm voice. If they get bad, you can pick them up, hold them so they have to look at you and tell them no biting again. That's what i did with my ducks... I haven't been bitten by a duck since they were babies...