What is a correct Amp for your RCD?
Nah, it should be OK if the RCD is rated more.  There are 2 different jobs being done here: 


Overcurrent detection (at 60 amp setting) - the MCB does that
Ground fault/Residual Current detection at any current - the RCD does that


The RCD might also do overcurrent protection, but if it does, it's redundant. 



So yeah, perfectly safe to use an 80 amp RCD. It can certainly handle any current a 60A MCB will let through. 

Obviously a 30A RCD doesn't have the internal wiring to survive 60A going through it, so that's a no-go.