Donald G. Phelps, the head of Seattle's three community colleges, will be the next chancellor of the Los Angeles Community College District and its nine campuses, officials announced last week. Phelps, 58, was chosen over three other finalists because of his record in helping students, his commitment to decentralization of power and his charismatic personality, said Hal Garvin, president of the Board of Trustees that governs the Los Angeles district. Garvin and other trustees said the choice was unanimous. The Seattle district has 20,000 students, contrasted with the 103,000 in Los Angeles community colleges. But, LACCD trustees stressed, Phelps presided over a system with problems very similar to the ones in Los Angeles, including declining enrollment, money shortages and even a controversy about headquarters that critics charged were too lavish.