Timing requirements to a part of a circuit

set max delay and multicycle path constraints are usually used to do different things. Max delay will make the fitter work harder, and multicycle path makes it easier for the fitter. They are not the same thing.

I still dont understand why you are doing this - if you are meeting constraints with just a clock constraint, why are you trying to overconstrain it? do you have problems with your design? The first method to fix re-occuring problems should be to fix the RTL code (add more pipelining) as this is the quickest and easiest fix.