I have been feeding my octos frozen shrimp from the market and they have been taking and eating them readily. I did catch some local crabs, thinking I would give them a treat, but I am concerned. If they like the crabs too much, will they stop eating the shrimp? I don't have the means to get them enough crabs (and of the right size) to feed them regularly. I also need to use shrimp bits for my experiment. Should I try the crabs? Or should I not to make sure they'll still eat the shrimp?

Just keep it mixed up. Don't feed them it crabs consistently or for a few days in a row because once they have eaten crabs a lot, they get attached to it, lol. It's probably their favorite mollusc. You should try other raw seafood as well so it doesn't have just 2 foods to attached to.