Sure, but in the end you pay (+ -) the same price for the yongnuo or nissin, and get added optical slave, shorter recycle times, automatic aps-c conversion for the flash zoom, things you don't get with the 550 EX
I agree that your Yn 560 will not play ball.
But beyond that every time you introduce a different brand, you multiply the chance of failure or at least inconsistency in how ETTL II is implemented.
You can still source pristine secondhand Canon 550Exs for equiv. of £130 which would give you a great 'Canon Wireless' network, offering both ETTL & M with HSS for either.