I think there would be no problem with multiple stablecoins or fiat-pegged coins as long as they were backed up by equivalent or higher amount of bitcoin.  such is the case of bitshares' bitusd and bitcny where these assets are backed by twice the amount in bts.  on usdt case, I am not sure if it is backed up by equivalent amount of btc.  if not, then big trouble will become imminent if majority of usdt holders will exchange it to btc.  that will surely lead to exchange collapse and another crypto catastrophe.