The Pentagon, following a tradition that is now kicking up an inaugural storm, plans to provide more than 900 drivers, escorts and military aides to VIPs in President Reagan's inauguraton Jan. 21 despite an opinion by Congress' investigative arm that there is no "legal authority" to do so.

An inaugural committee official, however, defended the practice, saying it is rooted in tradition and that the General Accounting Office, the investigative agency, had also said that military support "had been provided with the knowledge and approval of members of Congress over the years."

Yesterday, about 270 military officers got their first training for their controversial inaugural task, an etiquette briefing by White House military aides, who coached them on proper "body language," cautioned them to watch the edges of rugs so VIPs won't trip, and even explained how to extricate distinguished visitors from curious reporters.

"Tell [the visitor] he may have a phone call," the officers were told during the briefing at Bolling Air Force Base, according to a report made to Sen. William Proxmire (D-Wis.).

Proxmire, who had sought the General Accounting Office investigation after military aides were used in Reagan's first inauguration, criticized their use again as "insulting, ridiculous and a waste of the taxpayers' money."

"When we invaded Europe on D-Day, the final briefing involved 15 or 20 officers," Proxmire said yesterday. "Here we had 270 officers learning to check coats and use body language."

Proxmire also criticized the use of about $5 million in public funds for the inauguration, including appropriations for the Defense Department, the General Services Administration, which provides offices and equipment for the inaugural committees, and the D.C. government, for police and other city services.

In a time of huge deficits and spending cuts, Proxmire said, "We should not have the president, in an area over which he has complete control, go with this kind of wasteful expenditure."

Another inaugural official, committee spokesman John Buckley, responded that most events in the inauguration pay for themselves through the sale of tickets and souvenirs. "But even the good senator from Wisconsin should recognize that there are legitimate costs borne by government for security, crowd control and the removal of snow," Buckley added."

In 1981, the inauguration cost a record $16.3 million in private funds.

At that time, Proxmire charged that there were about $4 million in "hidden expenses" from public funds. He also berated inaugural officials for using more than 1,100 officers as chauffeurs and escorts to 274 celebrities and personal guests of the Reagans. Citing records he obtained from the Defense Department, Proxmire said aides were assigned to guests that included evangelist Billy Graham, former secretary of state Henry A. Kissinger and the Reagans' housekeeper, plus 42 entertainers and 48 governors.

At that time, inaugural Cochairman Charles Wick called the charges "unfair and beneath the dignity of a U.S. senator."

Yesterday, inaugural committee sources, sensitive to Proxmire's charges, said they are attempting to limit the number and the duties of the aides and escorts this time.

Indeed, the White House was so sensitive to the issue that White House counsel Fred Fielding was assigned to work out new "guidelines" with the GAO for the military aides' duties. The Armed Forces Inaugural Committee (AFIC), which coordinates the military's inaugural role, already is working within those guidelines, which will be announced soon, according to Navy Commander Greg Gagne, AFIC's spokesman.

Gagne said that although the numbers may be scaled back, as of yesterday, the use of 940 members of the military, including 500 as drivers, was planned.

About 78 military "aides" will be assigned to governors and members of Reagan's and Vice President Bush's families for "the period that the folks are here" during the four-day inaugural weekend beginning Jan. 18, Gagne said.

Another 114 "military escorts" will coordinate the arrival and movement to official events of various inaugural participants, including Cabinet members, Supreme Court justices and some entertainers and bands, he said.

Another group, about 225 "special coordinators" will help distinguished guests in arriving and leaving such events as the eight inaugural balls and two entertainment galas, he said.