Do not put anything in with your octo. The thing that goes around and people repeat is that "if you put an octo in your tank it will eat all your fish." This has certainly been true, and people have had their lionfish eaten by their octopus. However, if you put a bimac in that's 1" across, those other fish are going to be the predators.
The bottom line is if you put an octo in a tank with fish (even a damsel) it is going to have a very negative effect on the octopus. He's going to stay in his den all day, living in fear. You are not going to get the interaction or the viewing enjoyment that most octo-keepers value. These are sensitive creatures.
You're in a pretty good position right now because you aren't 100% certain if an octo is what you want to keep, but the tank isn't stocked yet. An octopus has a short lifespan, so if you order one when the tank is ready, he'll probably die by next summer or fall. At that point you can say "Hey, this was a real good experience, I'll get another" or "Gee, this isn't for me, I'll get some fish." OTOH, If you go ahead and get fish in the next few months, you're precluding yourself from ever keeping an octo in that tank.
Now that I'm done with that tirade, I would distribute the rock a little bit more, so it isn't in piles. You might also get some underwater epoxy from your fish shop and build some caves with it. A few PVC fittings (rinsed very well) would probably be good.
Dan