Opinion. I doubt that will be admissible.
If Zimmerman testifies at the immunity hearing, I think anything damaging he says on cross will be admissible at the trial, as a voluntary statement against interest.
I would think the officer issuing the capias request would be allowed to testify about the how or why he came to the conclusion to issue the arrest. I imagine the prosecution will ask him questions about his opinion of what led to his conclusion of a manslaughter charge and the defense will ask questions about statements that seem to contradict that charge.
I agree, if it were a general question to any law enforcement officer on the scene or involved in the case, but I would think Serino would be allowed to testify on the statement given he issue the capias.
IANAL, but I thought I recalled reading early on that if GZ testifies at the immunity hearing what he says can be brought up at trial only if he also testifies at trial. I do not know the pertinent statute and I imagine a lawyer familiar with immunity hearings could provide a quick answer.