Remember that they are only guidelines and that there are occasional exceptions to the rule. Males, even friendly ones, can over-power and frighten females. If a young, active male is your family pet, he would really enjoy having a male buddy who shares his enthusiasm for vigorous play. They usually make better mother substitutes than spayed females. Females, in general, are less accepting of newcomers.
Males tend to bond with each other unless both have dominant personalities.
Whatever the combination, a slow, systematic introduction process will help to ensure that the resident cat and the newcomer will eventually share the house amicably.