does skyfall of aerion trigger grand plaza with 7 units

Yes — but Grand Plaza's own trigger is what wins the game, not Skyfall.

Skyfall of Areion reads "My hold effects are also conquer effects, and vice versa" — "my" means the equipped unit, so it only rewrites that unit's own abilities. It never touches a battlefield's abilities (FAQ #288). Grand Plaza's "When you hold here, if you have 7+ units here, you win the game" is the battlefield's own Hold Effect, so it fires normally when you hold there with 7+ units, Skyfall or not.

Two related notes: the 7-unit check is part of the trigger condition, so you must already have 7+ units there at the moment you hold — the ability won't go on the chain (and no one gets a window to add units) if you hold with 6 or fewer (FAQ #10151). And the reverse doesn't work: conquering Grand Plaza with a Skyfall-equipped unit does not fire its "when you hold" ability.