One reason President Reagan may not agree to review the troops on horseback at Yorktown during the anniversary festivities next month is that he hasn't a thing to wear. As commander-in-chief of the Armed Forces, his well-worn jeans just wouldn't do. Not even with the snazzy patent ostrich boots inlaid with the presidential seal in gold. The troops will be in 18th century dress.

Were Queen Elizabeth coming she would wear the splendid scarlet tunic and tricorn hat and plumes based on the uniform of the Grenadier Guards in the 18th century. But it is Francois Mitterand, the president of France who will review the troops with the president. His problem is bigger than clothes. He doesn't ride a horse. "Maybe he could review the troops on a tractor," suggested one wag.

Paris designer Guy LaRoche may safely claim he was the first in the Paris haute couture to make knickers for women. Helene Rochas was among the first to wear them in green velvet 15 years ago. "To be too much in advance is like being too late; you must arrive on time," said the designer, whose boutique opened this week at the Watergate.

He has stuck mostly to knee lengths this season, rather than longer lengths which he likes only for sportswear worn with boots. "A woman who wears a long skirt, if she is not tall and thin and 20 years old, is not young looking," he said. "Besides, the leg of the woman is beautiful. Why hide it?"

LaRoche started designing with Jean Desses, the Paris couturier, when he was 19. After five years he moved to New York to learn about ready-to-wear from Seymour Fox, the coat manufacturer. He opened his own small Paris house two years later and it was such a success that he signed a contract to design as well for the New York firm Duchess Royale. In spite of a Life magazine cover and other recognition, the business closed because of family problems of the owners, and LaRoche vowed never to work again in America.

He now has a factory near Nantes with 700 workers that produces 300,000 garments annually. The factory could produce more, but LaRoche feels "more is too much. I don't want to destroy my name by putting my label everywhere." So he has bought the company Christian Aujard and will produce those clothes as well.

At the dinner at Jean Louis celebrating the shop's opening was the ambassador from Portugal and his wife, the Peter McCoys, the J. Carter Browns, the Arnaud de Bourchgraves,, the William Cafritzes, the Maurice Tobins, Helga Orfila and others. Said Ernest Marx, who with his brother has started many of the designer boutiques in this town, including that of Guy LaRoche, "This is the first designer who ever sent us a bouquet before the opening."

Yes, that really was former president Gerald Ford plugging the new muscular dystrophy clinic and Dash's Designer discount menswear shops on radio and television this week. "I would like everyone to know the vital importance I place on the work being done at Children's Hospital and its new Muscular Dystrophy Clinic; I share Dash's enthusiasm for helping those who cannot help themselves."

According to a company spokesperson, John Dash asked Republican House and Senate members, as well as the president and vice president, to do the spots. Neither Ford nor seven congressmen participating in the promotion, which continues on the radio through today, will receive remuneration.

New Yorkers could learn a lot from the way Washingtonians do their fashion shows, said Marielle McKinney at the Phillips Collection benefit this week: "The informality of the models walking through the museum is wonderful. In New York we tend to get stuck sitting on ballroom chairs."

A couple of New York models turned up their noses at Bill Blass' offer to come to Washington for $950-plus-fare -- far less than their usual $2,000 daily rate. But once they heard that Ronald and Nancy Reagan were going to be there, Blass' top assistant, Tom Fallon, got phone calls asking to be included, for $950, of course. The models never got to see the president -- they were kept from coming downstairs as part of the overall security. Eight of the 10 New York models were driven back to New York in two limousines after the show.

If you've missed the parade of designers going through this town this month, you'll have plenty of opportunity in October. Among the fashion designer events to come: Richilene at Elizabeth Arden (starting Sept. 30); Valentino show at the Italian Embassy (Oct. 4), Koos van den Akker benefit for the Washington Project for the Arts (Oct 5), Pearl and Albert Nipon benefit for Washington Antique Show (Oct. 9), George Stavrapoulos at Arden's (starting Oct. 20); Michaele Vollbract for Service Guild of Washington benefit (Oct. 26).

Halston, Geoffrey Beene, Pauline Trigere and Norma Kamali will discuss their designs with slides at the Washington Post symposium on fashion at Constitution Hall for the benefit of the Hospital for Sick Children on Oct. 1.