Please don't bother to try la.
As much as you wanna find justifications to believe that they can co-exist, it is a matter of time when the joint tenancy of tank will become only Single Tenant - Grouper.
Doesn't matter how big your Shrimp is .... you have not got a chance to measure how big the mouth of grouper can stretch.
I was at LFS Shop a month ago, While talking to the Shop Owner, I jumped because I heard a loud splashing noise from the tank next to me.
The Small Innocent looking Grouper WHACKED the BIG Damsel .... Couldn't swallow the Damsel in one bite, he chased and WHACKED again. Verdict: Gone .... the Damsel said 'Amen'.
LFS Owner told me that they have been co-existing for quite some time already.
So..... good luck to you.
P/S: In this case, the 'FIRE' will not kill the 'PANTHER' ..... because the Grouper hunts like a Panther but the Shrimp cannot Spit Fire at all ..