Cucumbers are good at cleaning. However, they can be toxic or venomous to the tank if they should croak. It'll probably be a safer bet to get a couple of fighting conch and some cerith snails.
Looks like you have algae growth starting on your sand. What kind of snails do you have?
Nassarius, BumbleBee and Cerith Snails would be great for attacking the algae problem. Also, a Fighting Conch would be good too.
Are you sure you want a big tank with nothing but a squid in there?
They're are very aggressive and will clean your tank of inverts. It'll get very expensive when feeding it.
BTW, dried cuttlefish is quite tasty :)
Any shrimp other than a Coral Banded will do. I'm not sure if any shrimp will have a symbiotic relationship with a Jawfish, I've really never heard of it. Then again, others may have seen this behavior.
My CC comes to the water line and folds back one of his arms. He likes to be hand fed Mysid Shrimp and Blood Worms. Also, I give him the occasional shrimp pellet and algae wafer.