I'm allergic to shellfish. Can I eat squid?

Maybe. About 10-15% of people who are allergic to shellfish are also allergic to mollusks (including squid). An allergist can help you find out. If you have not eaten squid since your shellfish reaction, you should be evaluated before trying because of the risk of reaction.
Maybe. Main familes of seafood: bony fish (cod), bivalves (clam), crustaceans (crab), cephalopods (squid), gastropods (snail), with cross-reactivity within a family but rarely between families. Though cross-reactivity between families is rare, it does occur, so you should consult an allergist before trying squid.