William F. Tuxbury Jr., a technology executive, was appointed president of Optigain effective Nov. 5.  . . .   Alexis P. Michas, managing partner of Stonington Partners Inc., has been elected to the board of PerkinElmer Inc.   . . .   BOC Edwards received the C. Sheldon Roberts Award of Excellence from Fairchild Semiconductor for the second half fiscal year 2000.