Six of the seven announced Democratic presidential candidates today expressed skepticism and criticism of President Reagan's decision to invade Grenada and suggested they would have operated otherwise had they been in office.

Appearing before 1,500 delegates and alternates at the New Hampshire Democratic convention, all of the hopefuls except former Florida governor Reubin Askew raised doubts about the Reagan action.

Askew said that, on the basis of present information, "I think the president was justified in what he did." Had the American students on Grenada been taken hostage by the Cuban-backed military government, he said, the United States would have faced "a much more difficult situation." Askew's response drew more boos than applause.

Sen. Alan Cranston (D-Calif.), answering next, was cheered when he said, "If we are going to attack every country where hostages may be taken, there will be a lot of invasions." Cranston said he did not believe the president's explanation that the students' lives were endangered.

Sen. Ernest F. Hollings (D-S.C.) rejected all of the explanations Reagan has provided--from the safety of the students to the buildup of Cuban arms to the request for help from other Caribbean nations. Saying the action had "recoiled" against the United States around the world, Hollings said a better policy would be to build "a Western Hemisphere common market" that would eliminate any need for military actions.

Former senator George McGovern (D-S.D.) said he would "terminate all U.S. military operations in Central America," and Sen. Gary Hart (D-Colo.) said the invasion raised the question in his mind of "what this president will do for a foreign policy when he runs out of Marines."

The two Democrats regarded as front-runners in this state's leadoff primary and elsewhere, former vice president Walter F. Mondale and Sen. John Glenn (D-Ohio), hedged their positions a bit.

Glenn said that in Grenada "American lives were apparently--I underline apparently--at stake. If they were, the president had no choice."

But later, in answer to the question of whether he would have ordered an invasion on the basis of the information Reagan has made public, Glenn said, "I do not believe I would. I hope there is more information still to come on what the president knew and when he knew it. The information we have so far . . . hardly warrants sending troops in."

Mondale said that "if in fact American lives were at stake, they should have been protected," and if the rapid buildup of Cuban military power were under way, "it should have been thwarted." But he said the administration's decision to bar American reporters from Grenada until the operation had achieved its main military objectives deprived Americans of information needed to make their own judgments.

The Democratic aspirants also focused their fire on the exposed position of U.S. Marines in Lebanon and said Grenada and Lebanon demonstrated what Glenn called "the chaotic approach" to foreign policy.

Although the forum focused on topical issues overseas, the Democrats also renewed their in-fighting on domestic policy and the tactics of the front-runners.

Cranston, in the most barbed talk of his campaign, said Mondale is "making so many commitments he has left us without any idea of what's really important" to him. The California senator said Glenn has failed the test of leadership because "saying that you can beat Ronald Reagan is not reason enough to be president of the United States."

All of the Democrats except Askew did agree on one thing. They signed a letter circulated by New Hampshire Democratic Chairman George Bruno, along with his counterparts from Maine and Iowa, defying the Democratic National Committee's edict on the early delegate-selection dates in those three states. They pledged to participate in the Iowa caucuses Feb. 20, the New Hampshire primary Feb. 28 and the Maine caucuses March 4, and to support the seating of delegates chosen on those dates.

All of the dates are outside the "window" set by the DNC, and its chairman, Charles T. Manatt, has threatened that the San Francisco convention may not seat the delegates picked on the early dates.

McGovern previously had supported Manatt's position, but changed his mind today when told that New Hampshire legal authorities had ruled this week that the primary here must be held Feb. 28, a week earlier than DNC rules allow. Glenn's position had been unclear, but he agreed to sign the letter.

Askew told the convention today that he had no objection to the early dates in Iowa and New Hampshire and would compete in those states. He refused to sign the letter, he said, only because he could not support Maine's claim to an early date for its caucuses.

In Washington, DNC Director Michael Steed said Manatt will hold firm in his effort to enforce the party rules as they stand--setting up a possible confrontation between national party officials and those from the three states.

As of now, Iowa, New Hampshire and Maine do not have any plan before the party's compliance review commission, he said. Steed said the following steps lie ahead:

When the states submit their plans, DNC officials will see if the plans comply with party rules. If the three states are judged to have not complied, the DNC will issue a final order of compliance. If the states proceed to conduct their caucuses and primary on dates not approved by the DNC, Steed said, delegates chosen by those caucuses and primary will not be seated at the convention.