Small communities like Gallup, N.M., and Greenwood, Miss., should flourish in coming years as people flee both big cities and suburbs while businesses seek new markets, says population researcher G. Scott Thomas.

Although attention and headlines focus on the major metropolitan areas, "micropolitan" America may become increasingly popular, Thomas says in the May edition of American Demographics, a magazine focusing on population issues."An increasing number of people are growing tired of central cities where crime rates are getting out of sight," Thomas said Tuesday.

"Smaller cities are as influential in their regions as metropolitan areas are on a larger scale," Thomas said.

Thomas defines a micropolitan area as a county with a population of at least 40,000 people, with a core city including at least 15,000 - but not a part of a metropolitan area. Metropolitan areas, which are officially designated by the federal Office of Management and Budget, are economically related counties generally focusing on a central city of 50,000 people or more.

He reports that between 1980 and 1986 some 83 micropolitan areas grew faster than the nation as a whole.

Leading the small city growth was Rio Rancho, N.M., which increased by 48.6 percent over the six years. Helping was that city's nearness to fast-growing Albuquerque, Thomas reports.

Like metropolitan areas, the fastest growing micropolitans tended to benefit from Sun Belt locations. Of eight that increased by more than 25 percent over six years, seven were in the Sun Belt.

Quick growers between 1980 and 1986, besides Rio Rancho, were Bullhead City-Lake Havasu, Ariz., 37.1 percent; Vero Beach, Fla., 35.2; Hilton Head Island, S.C., 31.0; Huntsville, Texas, 29.4; Myrtle Beach, S.C., 28.8; San Luis Obispo-Atacasdero, Calif., 26.5; and Fairbanks, Alaska, 25.3.

Torrington, Conn., had the highest per capita income of the more than 200 micropolitan areas studied by Thomas. Torrington reported incomes of $13,381 per resident. Fairbanks was second at $13,079, followed by Bartlesville, Okla., $13,035; Key West, Fla., at $12,319; and Vero Beach, Fla., $12,155.

Many micropolitan areas are the homes of major universities, Thomas observed, resulting in several communities with high median educational levels.

Corvallis, Ore., and Pullman, Wash., led that list with 14.2 years of schooling completed for the typical resident aged 25 and over. Bozeman, Mont., was next at 13.5, followed by Ames, Iowa, at 13.4. Others with medians of 13 years of schooling or more were Ithaca, N.Y., and Logan, Utah, 13.2, and Manhattan, Kan., 13.0.

Here is a look at the top five micropolitan areas in other categories measured by Thomas:

-Oldest median age: Vero Beach, Fla., 39.4; Prescott, Ariz., 39.0; Hot Springs, Ark., 37.2; Bullhead City-Lake Havasu, Ariz., 37.0; Pottsville, Pa., 36.8.

-Youngest median age: Gallup, N.M., 22.0; Hinesville, Ga., 22.1; Mount Pleasant, Mich., 22.5; Manhattan, Kan., 22.6; Radcliff-Elizabethtown, Ky., 23.0.

-Percent black: Greenwood, Miss., 59.1; Orangeburg, S.C., 56.0; Greenville, Miss., 55.6; Selma, Ala., 54.6; Roanoke Rapids, N.C., 47.1.

-Percent hispanic: Alice, Texas, 67.2; Del Rio, Texas, 62.9; El Centro-Calexico-Brawley, Calif., 55.8; Carlsbad, N.M., 30.7; Roswell, N.M., 30.6.