Yes. Squid is protein and water with essentially no carbohydrate, so it grades clean on every FODMAP group: no fructans, no galacto-oligosaccharides, no excess fructose, no polyols, no lactose. The FODMAPs in a calamari dish come from what is served with it, typically a garlic aioli, an onion-heavy marinara or a batter, not from the squid.

It grades moderate, not high, and it does not liberate your own histamine the way some foods do. Squid is usually frozen quickly on the boat, which keeps histamine low, so the risk sits with what happens afterwards. Seafood that has thawed slowly, sat in a warm kitchen or been held on a buffet accumulates histamine, and no amount of cooking will remove it once it is there.

Not without medical advice. Shellfish allergy splits into two groups, crustaceans (prawns, crab, lobster) and molluscs (squid, octopus, clams, mussels), and they are labelled separately for that reason. They share the muscle protein tropomyosin, so cross reactivity is real and some people react to both, but many people allergic to one tolerate the other. This is a question for an allergist and a proper test, never something to work out by trying a plate.

No. Squid itself is naturally gluten free, but calamari is nearly always coated in wheat flour or semolina before frying, and restaurant fryers are shared with breaded and battered items. If you need gluten free, grilled, braised or salt-and-pepper squid cooked in a rice or corn flour dredge is the version to ask for, and worth confirming with the kitchen.