Well, there were actually two Mr. Handfords: the first portrayed by Leonard Jackson for his debut season, then David Langston Smyrl played him until 1998. Jackson's Mr. Handford was, indeed, generally depicted as kind of a cranky, grumpy old man, but then again, the character was supposedly a retired firefighter, and you know rescue workers have to be tough around the edges, and not let anything phase them. David L. Smyrl's Handford was a bit more mellow. Perhaps Mr. Handford was meant to be someone who is not exactly a "people person" (like the way we first got to know Shrek).
As for Asian-American characters, no, Celina was not the first, though she WAS before Alan. During the late 80s and early 90s, Lord Of The Nerds himself, Gette Watanabe was Hiroshi, the Japanese-American artist who could be see with his palette and easle just about anywhere on the street... even Oscar's can. But it is nice that they finally found a steady, regular Asian-American, and I've liked Alan very much since he came to the street over a decade ago.