My Mantis is much smaller a peacock (21/2 inch G. Smithii). Based on my research he shouldn't bother any fish above half his length. I also have a fairly large CUC/food bank for him, and I feed regularly. That said, there is a risk, and that is why I started with a clown. I had to balance a test fish as a fish I wanted to keep, but one that was not really expensive.
What can I say I want a full tank not just a mantis.