I want to know, if I buy a Yongnuo YN600EX-RT flash, is it compatible with a Canon 550D ?
Yes it is compatible.
Of course the combination would work just fine in manual mode, but you likely are interested in some automated(evaluative) flash capabilities.
The Yongnuo YN600EX-RT flash is compatible with Canon E-TTL / E-TTL II and it is also compatible with the Canon 2.4 GHz Radio System (allowing you to pair with a 600EX-RT or ST-E3-RT). So in essence you really lose out on almost nothing when comparing this to the vastly more expensive Canon options (there are some minor differences for those that are particular).