In what has become the largest strike by city employees in Canadian history, 15,000 indoor employees of the City of Toronto yesterday joined outdoor city employees who have been on strike for more than a week over failed job security negotiations. Among the workers now on strike in an effort to reduce the probationary period for obtaining tenure from ten years to six years, are swimming pool attendants, day care center workers, animal shelter employees, sanitation workers and public health inspectors. Toronto Mayor Mel Lastman appears to have taken a hard-line approach to the strike, encouraging the Ontario provincial legislature to pass legislation forcing city employees back to work, and using the possibility of such a measure to pressure workers to end the strike.