on hashflare, there are just 4 algorithms supported SHA-256 (BTC) , Script, ETHASH, EQUIHASH (ZEC), indeed you cannot change the algorithm. if you start for example with BTC and then you want to mine ZEC, but what you can do is to use your mined BTC to buy hashing power for another algorithm.
There are some other mining platforms, but I don't know what they offer at the moment.