Hi Molson...welcome to Tonmo!
(don't worry if some of the residents appear a little strange...I, of course, am perfectly normal...
You may not even have to use fish, depending on your preferences. If you want your tank to be an octo tank eventually, you'll have to remove those fish at some point down the road, or accept their ultimate fate as dinner!
Further, you'll need to consider your initial selection of fish--some are potentially threatening to an octo.
I started out with invertebrates...some urchins, brittle stars, snails, etc. I decided to get a "lettuce slug" nudibranch (a type of sea slug, if you're unfamiliar with them) as a "test animal." I figured it would be relatively delicate, and as close to an octopus (they are all related--snails, slugs, clams, oysters octopus, squid, etc.) as I could get for a low price. He did just fine in my tank...uh...that is...until I accidentally removed him when pruning my macryphytic algae...
Well, he WAS fantastically camoflagued... Anyway, after that, I knew the system could handle an octopus.
Biggest secrets to my success were research, the use of plentiful, high-quality live rock, and a general conservative approach--I cycled the tank longer than necessary with my live rock and sand, just to be sure, and I added animals SLOWLY, one at a time, with a very low overall bioload--the tank could easily handle twice what I've added. With this approach, I found keeping the tank to be EVER so much easier than I thought it'd be.
Now, with all that said, I suffered a major tank crash a bit more than a month ago. I had (for my and the tank's safety) plugged all the equipment into a GFCI outlet, which tripped while I was on vacation for a week during thunderstorms. This killed some of my animals, including my octopus, up to that point a happy resident for about 5 months. If I'd had more experience as an aquarist, I'd have known this was a risk, and would have taken appropriate measures to prevent it. I also regret not setting the system up with a sump from the beginning, which I'm now building. So, don't feel silly about wanting to take your time. You can probably have an octo on a relatively short schedule, but it will definitely raise your risks of an accident here or there, which could prematurely scare you away from this wonderful hobby.
Now, one more little secret...the reason I spent so much time talking about the way I
set my tank up is...I don't know a thing about keeping vertebrates!
But don't tell anyone, okay?