Can the bitcoin network hashrate be used for a 51% attack on other coins?
You could easy manipulate a pool to wreck havoc on other SHA256 coins if you owned it or managed to gain access. You don't even need to be a large pool, some altcoins are really small and vulnerable. It has affected other coins before, where a pool or a large miner created trouble solely by mining a smaller altcoin.

Feathercoin hit by massive attack

Only thing that prevents this regularly happening is that is isn't profitable to attack small crypto-currencies and in fact devalues whatever you make in the end.