Are you brining the fish? The reason I ask is that when I have done fsih, I brined it. Then, I leave it in the fridge on a rack until a pellicle forms which looks like a wax coating.
Once it has formed, I smoke it and the pellicle is more pronounced. Thus, it seems to seal the fsih and in your case I would think you would get less of a mix. In any event, I don't know what the harm would be.
If it were me, I would put the fish on top and the others on the bottom if given a choice. Plus, since I don't know how much fish you have but I assume it's not as much as the beef/pork, you may want to put it in later during the smoke when the meat pores are more likely to be closed.
I'm just thinking out loud here as I've never combined them but it's a good question.