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.
Yes i've noticed that BTS have the whole stablecoin approach differently, interestingly enough bitCNY overtook TUSD in volume just afew days ago! Indeed it's a quite controversial topic and i'm afraid it will end quite badly, I think it's best to stay away period. The collapse of these assets could have a huge effect on crypto as a whole is the problem.