The president of one of four electrical firms accused of bid-rigging on government contracts pleaded guilty in federal court in San Jose and agreed to testify for the prosecution. U.S. District Judge William Ingram placed Theodore Strotman, 58, head of San Jose-based Howard Electric Inc., on three years' probation, fined him $10,000, and fined Strotman's firm $110,000. The three other firms charged with violating federal antitrust laws are Rosendin Electric and C.V.E. Inc., both of San Jose, and Steiny & Co. of Los Angeles. Jonathan Howden, an attorney for the Justice Department's antitrust division, would not comment on the possibility that Strotman's testimony might lead to further indictments.