Nope, the ones in the article are ASICs, so specifically for bitcoin. There are GPU based ones but the number of coins that you can mine with them is dwindling. You could probably reuse the beaglebone black or whatever little auxillary computer they're using but it's not really worth it. It's just e-waste.
Depends what they actually are, some 'mining rigs' are just PCs with lots of beefy GPUs, so yeah.
Others are FPGAs programmed to compute the hash for whatever cryptocurrency (e.g. BTC is `sha256(sha256(x))`); they can be re-programmed to do something else instead.
Others are ASIC (Application-Specific Integrated Circuits) based, physical chips designed to do nothing else but compute those hashes. So essentially 'no', but 'yes and they'd be very good at it' if you for some reason had another use for that particular hash function (lots and quickly).
That's also a progression of cost & performance, by the way. As the difficulty increases GPUs get slower at finding whatever coin, and people (pay a bit more and) move to FPGAs, and then again to dedicated ASIC hardware.
Not really they're extremely specialized chips that only do lots of hashes fast. They can't even be used for an different blockchain unless it's block hashing is similar enough and uses the same algorithm.
