If you want tankmates find some cool seastars. I would avoid any large carnivorous stars though. Linkias, serpents, and brittle stars (except green brittles) are fun to watch and sometimes really colorful. You want to avoid anything that's going to be potentially aggressive or potentially octopus food. Believe me, if it's an O. hummelincki like swf.com usually gets, the octopus alone will be all the excitement one tank can handle.
I have no idea, but I wouldn't put anything in a tank with an octopus that I didn't want it to eat. If you just want to keep it safe, maybe in a sump would be better if you have one. I'd certainly think if it hatched the octo would be pretty likely to eat it, I have no idea before it hatched. You could try keeping the egg in a critter keeper or similar, but octos are famous for being able to open things and crawl through small holes...
I don't expect that you'll get a specific answer, but "better safe than sorry" is usually a good rule unless you have some compelling reason to want to have the egg with the octopus. Of course, if you decide to bet your shark egg on the unknown odds of the experiment, let us know how it turns out... but even then "it worked once" is not really reliable even if it does... sometimes octos can share space with fish, but often they can't so we've occasionally had people say "I've kept octos with fish with no problems" but overall, more often than not it leads to some sort of problem, so we usually say that octos are best in a species tank with only very innocuous tankmates, for the good of the octo and the tankmates, since frequently one or the other will either become lunch, or will be so stressed that it will not be healthy.
the egg wont hatch for another 4-5 months but your right i rather be safe than sorry so i'll just keep it in my other tank. i've seen people keep a large octo with 2 leopard sharks in a 180 gallon tank so i wasn't sure if i could do the same with a banded cat shark.
I know of at least one case where a small shark killed a small octo, and the famous Seattle Aquarium video shows that at least sometimes a large octo can kill several medium-sized sharks... I would certainly discourage keeping any octo with any shark.