With the interest Zcash is getting I highly doubt that any single miner will be able to pull off a 51% attack, that would mean a tremendous hashrate controlled by a single user. If there is such a user he would most likely be willing to mine and profit from it than using it maliciously. I'm actually yet to see a coin being destroyed by a 51% attack and double spending... though some smaller have suffered from loses from such attacks. It is not like you can just point out your Bitcoin mining farm to mine Zcash, because ZEC uses new and different proof of work algorithm and you can only mine it with CPUs and GPUs. Renting all available hashrate from a service such as NiceHash will also not be enough for 51%... and before trying to double spend coins you need to actually have some, so you need to either buy or mine a significant amount first.