Well I don’t know about DOSing mining servers on a cloud mining service, but assuming it’s technically possible, you would have to react pretty quickly I guess. You wouldn’t want to postpone executing your malicious plan for too long anyway, because keeping up the attack gets pretty expensive pretty fast. You probably would have to do two trades together worth more than 700K in one hour. That seems to me like much, but not impossibly much.
As to what I’ve read it is quite impossible to get the required hashrate in the first place though, there’s not enough supply to go around. But smaller blockchains like the recently attacked Bitcoin Gold are more vulnerable, due to the amounts of available rentable hash power for Bitcoin. Those chains therefor seem to be more at risk. Hopefully it’s balanced out because there’s less to earn there as well, making an attack unprofitable.